US2465962A - Protection of bore surfaces of guns - Google Patents

Description

.March 29, 1949.

H.B. ALLEN ETAL PROTECTION OF BORE SURFACES OF GUNS Filed April 28, 1945 l 2 Sheets-Sheetl l kll` Nital H Emth MMMMMWM@ www 2 Sheets-Sheet 2 H. B. ALLEN I'AL PROTECTION OF BORE SURFACES OF GUNS March 29, 1949 Filed April 28, 1945 Patented Mar. 29, 1949 2,465,962 PROTECTION OF BORE SURFACES OF GUNS Henry B. Allen and Pa., assignors to Nicol H. Smith, the United States of America Philadelphia,

as represented by the Secretary of War Application April 28, 1945, Serial No. 590,846

6 Claims.

The invention relates to means for protecting and prolonging the life of the throat and rifling in ordnance pieces, and seeks particularly to present means especially applicable to use in current gun models with little modification. It is well understood that erosion at the forcing cone and adjacent riing by the highly heated and rapidly moving gases at the high pressures involved in the gun chamber, and the diversion and compression thereof in the throat marks the beginning of deterioration of bore faces and riling. This is soon followed by a condition whereon firing shock and shearing effects occur by the projectile moving without rotation from the chamber over a part of the bore where riiiing has been destroyed, and then encountering partly eroded rifling after considerable acceleration. This may then become the major factor in deterioration of the bore.

An object of our invention is the provision of an effective and thoroughly reliable method for protecting the bore surfaces of guns from influences of a wide variety which otherwise lead to erosion and untimely wearing out of the gun.

A further object of our invention is the provision of mechanically simple and inexpensive means for protecting the bore surface of a gun from wear and deterioration by propellant explosives, which may be included in the ammunition as loaded into the gun incidental to simple firing routine.

A principal aim of the invention is to provide an expendible inexpensive facing or chambering element in the round itself which will afford a surface material suicient to withstand the erosion manifest in firing the single round at least, this material extending over only the critical area where erosion has heretofore been observed to start in conventional bores, so that the further deterioration by shock and shear after initial erosion will be avoided, as Well as erosion otherwise materially reduced.

A still further object of our invention is the provision of a gun which is adapted to receive chambering rounds of ammunition including bore surface protective means.

Other objects of the invention will be obvious, and in part pointed out hereinafter.

The invention accordingly consists in the individual elements, combination of parts, composition of materials, and in the several steps, and the relation of each of the same to one or more of the others as described herein, taken in the light of the accompanying drawings: and set forth in the following claims.

In the drawings herewith Fig. 1 is a longitudinal sectional view of a round of ammunition embodying certain features of our invention;

Figure 2 is a detail elevation of the sleeve;

Figure 3 is a fragmentary section of a round showing the standard .50 caliber bullet and a short sleeve;

Figure 4 is a fragmentary longitudinal section of a gun barrel, more particularly of chamber and bore portions;

Figure 5 is a longitudinal section of the barrel and round illustrated in Figures 1 and 4, the representations of land forms being purely formal;

Figure 6 is a similar View showing the use of a short liner sleeve with modification of the barrel;

Figure 7 is a similar view showing the invention adapted to the current ball ammunition.

As conducive to a clearer understanding of certain features of our invention it may be noted at this point that gun erosion is a complex phenomenon involving a combination of numerous effects; predominantly thermal effects, chemical and metallurgical effects, and mechanical effects. all of which contribute, as the result of gun use, to shortening the useful life of the gun. The individual or combined intensity of the various effects is dependent upon such factors as gun material, type and quantity of powder and type of projectile employed, rate of fire, and velocity of projectile. Gun erosion is most severe within an area beginning at the forcing cone and origin of rifling and extending some few calibers along the gun barrel toward the muzzle.

As the result of high temperatures reached by the powder gases, a thin layer of metal at and near the bore surface of the gun is rapidly heated to a temperature which, under certain circumstances, exceeds melting range of the metal. Even if the bore surface is not melted, the temperatures reached are such as to reduce the strength of the steel and facilitate its displacement. In cases where gas leakage occurs the material may be simultaneously melted and carried away to a marked degree by scouring action of the escaping gas.

At the high temperatures attained by the powder gases the products of powder explosion are chemically active. Furthermore, the raising of the bore surface temperature increases the chemical reactivity of the gun material. As a result, there is a very complex set of reactions between the gun material and the powder gases. The importance of chemistry in the erosion process has not as yet been fully evaluated, but the existence of a considerable variety of reaction products has been established.

the beta allotropes are exceeded. This layer has... lower strength, probably a lower melting ,e point.

due to chemical effects, and seemingly is less resistant to erosion than the underlying steel.`

There are several mechanical eifects which are known to contribute to the erosion process. In

the first place, simple frictional or yabrasive wear by contact of the projectile is a factor, the importance of which depends upon gun and projectile design and materials. When the steel surface of a projectile Ybears upon a bore surface of steel, galling may occur in addition to abrasive wear, with consequent removal of small masses of metal from the bore surface. Both the abrasive wear and galling tendency are aected by the surface temperatures attained in the bore. The temperature at the point of rubbing is influenced by the instantaneous heating effect caused by friction.

It is also to be noted that in conventional guns bore diameter is increased at the bore surface as a result of intense surface stresses developed by engraving. This is particularly `true under rapid re conditions, when the gun is quite hot. Metal in the lands is displaced radially outward, forward, and circumferentially sideways. Even though no metal is actually removed, the lands are gradually obliterated, by beingchammered, swaged or pressed down into the surface from which they at rst rose and by being broadened and extended by attening. Any downward radial ldisplacement of that portion of a land under the projectile rotating band is accompanied by an upward displacement of the land surface ahead of it. In effect awave, consisting of a depression of the land material near the origin of riiling and a raising of material in front of this point proceeds down the bore as ring progresses.

An outstanding object of our invention, accordingly, is the provision of a method forL conditioning gunsso as to improve gun resistance to plastic deformation and toerosion normally producedl by thermal, chemical, metallurgical, and mechanical effectscombined or separatelyincident to firing of conventional ammunition.

Referring now more particularly to the practice of our invention, we increase the useful life of any of acwide variety of smooth bore or rifled guns of different sizes to be employed for automatic or single shot fire at high or low velocity, through the use of thin sleeves which provide a protective covering for the bore surface of the gun over at least that part where most severe erosion and deformation commonly occur. The sleeves preferably are affixed as individual elements to the cases of fixed ammunition to be red in the gun, or-illustratively--may constitute vintegral extended portions of such cases, so as in either event to be insertable or ejectable with the case in the normal firing function of the gun.

In order to make sleeve protection of the bore surfacespracticable at the breech end oi the bore we prefer Ato-provide a gun which has a slightly enlarged bore including the forcing cone .toward the muzzleY and extending for a distance suiicient to permit and ensureproper chamber,-

ing of the ammunition including the sleeve. Aside from the feature just mentioned, the gun barrels conveniently are wholly of conventional contour along the bore surface. Preferably, but not essentially, the bore surface, illustratively at least that part immediately in advance of the enlarged bore portion and continuing forward to the muzzle, is of chromium or other erosion-resistant material.

In Figure 1 of the accompanying drawing there is represented at l0 an illustrative round of case ammunition which we employ in the practice oi' our invention. The particular round shown corresponds Ato one of artillery type used by us in a Y.50 caliber ried gun, although the invention may usual ,primer centered lin the base thereof. 'ThenA forward. part of the Vcase comprises a tapered shoulder iportionlZ which terminatesy in a conventionalopen neck `I 3.A An artillery type projectile or; bullet I4 (of approximately 0.5 inch diam-` eter)v fits rmly into theopen'neck and has an interposed rotating band I5 set within-and eX,-

tendingforwardlyof the outer edge of the neck..

Al thin sleeve I6, illustratively madeA of drawn. soft steeltubing and having a `wall thickness of about 0.010 inch, is mounted over the case neck..

vIt is :formed witha large diameterrear part l1,

and tapers down before-but. not necessarily enr., gavging-the projectile rotating band .as at I3.,

so asv to provide kforwardly ar slightly reduced cylindricalforward part I9 having an inner diam.- eterwhichis slightly more than 0.500 inch. On

this .di'ameter'the sleeve `extends from near the rotating bandiorwardly over the surface of the projectile to .a distance somewhat in advancev of.

the projectile nose in this particular form. The

sleevefforthe,` sake of simplicity ofY assembling, preferably is madev so as to be slipped readily into this position onthe round after manufacture of the standard prior round... In order to ensure proper'withdrawalfof thesleeve when the car- .tridge is extracted by breech mechanism after the round has been flreduwe provide severalperipherally spacediindentations orperforationsl in the sleeve nearthe Abase of the same prefer:

ably located so as to be rearwardly ofr theY bullet g asin Fig, 1'. The holes may ,be circular, orrectangularas inFigrZ, to afford better keying with the cartridge. When, the. round Vis ined, metal `of.

the case neck is forced permanentlyinto the per: forations by powder gas pressure within thecase,

to effect apositivelock betweenthe case and the sleeve.

For Vfiring the ammunition, We provide a gun havinga ried barrel 2| :(see Fig. el of the drawing) which is modiedadjacent the` chamber to. vaccommodate the sleeve element. I 6.; 4The `forward part or neck2'l ofthe chambentheforcing cone 24 and adjacent relievedsectioni 22 .of the ried bore,are enlargedzn radius as `compared with conventionalfguns by an amountI whichV is substantially equal tothe. thickness'fo'f the sleeve- It 4'will be noted that .the rifling'zaextends from the lfore@- ing -cone A2l! Aforwardalongthefbore surfacev tof the enlarged bore. section22, v.then is stepped or;

inclined inward at 25 and continued conventionallythrkough the `advancedportion. 2,6 'lof the bore,

s which may be of conventional caliber and length.

With a round of sleeved ammunition properly chambered in the modified gun, as illustrated in Fig. of the drawing, the bullet rotating band I5 is disposed immediately in back of the forcing cone :24. The forcing cone and rifled bore surface extending therefrom are separated from the bullet by sleeve I6 which rests on or is substantially geometrically congrous with the bore surface, including the forcing cone and land tops 28 of the enlarged bore section. At its advanced end the sleeve adjoins the .50 caliber section of the rifiecl bore and is substantially alined, inside, with the land tops of the same.

When the gun is fired, using the round of Fig. l, the bullet I4 carries rotating band I 5 past forcing cone 24, at which time engraving of the band by means of the rifling begins through sleeve I6. The sleeve also is actually engraved by extrusion into the grooves along with the rotating band. Before and during this period metal of the shell case II is forced by powder gas pressure into the perforations 20 at the base of the sleeve for locking or keying the two elements together. The sleeve protects the underlying metal of the gun from erosive effects of the hot powder gases as well as from mechanical effects set up at the forcing cone and by subsequent engraving: also by insulation of heat from the tube material. After the bullet reaches the end of the sleeve, additional engraving of the band or bullet case which may be necessary takes place in the advanced bore section. After firing, the hot case and hot sleeve attached or keyed thereto are ejected from the gun by the usual breech mechanism operating on the case (removing a substantial amount of heat which was prevented from reaching the barrel), followed by loading, firing and ejection of successive similar sleeved rounds as desired. The overall effect is to relieve the gun bore surface of exposure to the most severe conditions which normally contribute to erosion and plastic deformation, with the result that useful life of the gun is considerably increased.

While it is desirable to extend the protection of the forcing cone 24 and rifling in the bore as far toward the muzzle as practicable there is a limit to the practicability of extension of the sleeve and also a point of economic balance and compensation between the benefit of prolongation of life of the weapon by this method and cost of modified ammunition and gun, modification of handling routine, and modification of belts and firing mechanism. The sleeve of Fig. l has been found practicable` but in order to limit expenditure of material and for other reasons it has also been found very beneficial to use a sleeve which does not extend beyond the bourrelet or possibly the beginning of the ogive. This is especially satisfactory when the bore forwardly of the sleeve I6 (or I6 hereinafter described) is faced with a wear resisting material. There is a peculiar coordination of these two features practicable which is distinguished from merely facing the gun surfaces with wear resistant material. Thus, the hard materials available and suitable for the use are scarce and expensive; they can only be practicably applied in thin facings; facings which are effective to assure an adequate length of service to bore faces at a distance from the conventional chamber are incapable of preventing deformation of underlying material of the gun tube at the forcing cone, due to high temperatures effective through an otherwise adequate facing, and would permit plasticising and deformation of such underlying material. But with the use of the short sleeve (Fig. 3) and its repeated renewal on each round, the lowering of temperature of the gun tube material by absorption and impedance in the sleeve and by conduction to the fresh sleeves; a moderate degree of insulation, and the dissipation of heat by the removed sleeves; as well as protection of the overlying material of the gun tube against erosion, enable a very extended use of the gun with immaterial depreciation of the forcing cone and adjacent rifling, and practically no wear of the forward riing, when the thin facing is used from the chamber forward.

In Figure 3, the sleeve and parts corresponding to those already described in the sleeve I6, have the same reference characters with the addition of a prime mark. The cartridge case is unchanged, and the rear larger part I 'I' of the sleeve is of the same thickness, diameter and length as the part I1; the taper I8 is the same as the part I8 of sleeve I6, but the forward reduced part I9' of this sleeve, while of the same thickness and diameter as in the forward part of the sleeve I6, is only about one halfor less than half-the length of the forward part I 9 of the first described sleeve. This figure also illustrates a conventional soft jacket standard boat-tail bullet I4', familiar in the type of arm indicated. Its jacket 29 serves in place of the rotating band I5, and is engraved by the rifling of the barrel, with the same effect.

Although marked increase in gun life is obtained without shielding that section 26 of the bore surface beyond the protective sleeve I6', we prefer to face this section, including land and groove surfaces, with erosion resistant and corrosion resistant material such as chromium, or other material, as, molybdenum, tungsten or tantalum, in normal or hardened condition. The

advanced facing thus constituted, preferably, andy 4for best results, may be electro-deposited or otherwise formed to such thickness as will prevent the formation of heat-transformed layers in the underlying gun metal. By using the advanced facing, the length of sleeve on the case ammunition may be shortened as compared with the length desirable when the lining is omitted. It will be understood, however, that the shortening of the protective sleeve is not essential, and also that a shortened sleeve may be used without necessarily facing any parts of the gun bore with harder metal.

Dlustrative of that embodiment of our invention which includes the advanced lining or facing, a gun barrel 30 riiled from adjacent the forcing cone 3| to the muzzle is shown in part in Fig. 6 of the drawing (the muzzle being omitted). It has a thin, adherent, electrolytrically deposited chromium plate or facing 32 on the bore surfaces, extending continuously from the shoulder 34 of the chamber and across the forcing cone 3I beneath and adjacent the part I9 of the protective sleeve I 6' of the cartridge case I I, and covering the advanced bore surface 26 up the barrel for a substantial distance, or all the way to the muzzle (not shown). A thickness of chromium plate ranging from 0.003 inch to 0.006 inch or more preferably is used as the facing 32 to prevent heat transformation or other objectionable modication of the underlying steel at least in the barrel beyond the sleeve IS; and in such thickness the chromium of the thickness named, in automatic arms of considerable range of caliber, and specifically the .50 caliber Browning machine gun is found to remain intact over an exceptionally long period of use. Y

In this-ligure ,(6) the snortsleeve; l this shownff as in Fig. 3. The chromium plate 32 in the ad, vanced bore section, deposited over the rifled steel, provides the standard caliber finished rifled bore surface.-

While the whole bore of the barrel, 30 upon which the facing 32, is to be applied must be tooled to, a larger than standard radius for a givencali'.- I

must'be also an allowance for the thickness .of the.

sleeve inthe width of the lands in the part 35. Thus, if the lands inthe advanced bore 26 are to have a given width of finished land face v36,

the lands 31 as tooled vin that partv should bev approximately 0.006 pinch narrower wherethe facing y32 'is tobe 0.003 inch thick. In the en larged bore 35, however the tooled lands 38 should be approximately 0.026 inch narrower, where the rotating ring is to require no ,material further engraving afterleaving the sleeve. If it is desiredthat there shall be some small degree of further engraving performed ,by the bands of the forward bore 26' the lands 38 may be reduced in height, thatY is, their radius increased somewhat x furtherthan the 0.013 allowancelnecessary for they sleeve and facing aloneand the reduction of widthA of the tooledlandsu38 may be, say 0.027,-V

necessary. In such case also, .while the sleeve I6 is not shaped to rifle form on firing, as in the previously described gun, it will protect. thehoreH from the severest erosion, corrosion, and plas.- ticising effects heretofore incident to rapid fire of the gun, so that the advanced bore lands 43 will be subjectto less deterioration. And if the advanced bore d3 and relief 42 are faced as indicated at 45, its life is extended many times that of the conventional barrel in such guns.

Usingthe round of Fig. 6, the round is. cham-1 bered in the barrel so that the rotating'band v is directly behind the forcing cone 3l, with the sleeve I6' taperingat I8' with the forcrig'cone,

to the smaller diameter i9 over plated rifling of the relieved bore section 22'.

When a gun comprising barrel 30 of Fig. 6, with chambering: case ammunition of the :character there shown, is fired, the artillery type bullet I4 carriesrotating band I5 past forcing cone 3 I, and

' engraving of the rotating band by the lands of instead,;ofp0.026. Theallowance of- 0.026 inch,A

would result inthe interior surfaces of the sleeve part I9 being-shaped by the band I5 to thesame f contourand measurements as ,the interioriinished faces of the riingofthe advancedborel 26. The-greater allowancementioned (0.027 inch) will insure that the material of the sleeve pressed# into the riing of the relieved lpart will havean inner cross section surface form'the same as that. of the, advanced bore,'butY differing lsufficiently in dimension at Ithe inner faces of the sleeve lto re-l quirea lslight additional engraving of Athe band I5 as thelands of the advanced part of the barrel y are4 encountered. This construction, requiring relief of the neck for the facing 32 inaddition to the allowance for the sleeve, results, in r,shortening the shoulders 34 of the chamber as tooled but still affording an adequate stop for engaging; the

shoulder I2 of the cartridge. The step 39 lne--l tween .the enlarged bore v35 and the advanced bore 26. may be sloped as before at4 25in Fig. 4orV otherwise shaped as requirementsv dictate.

It isA believed that the iinishedsurfacesxof the relieved part 35'may besuch that a close aline.- ment lof all internal surfaces of the sleeve (as formed when pressed ontoyand into'the rilng of the relieved part 35) with corresponding surfaces i of the advanced lbore 26',may be attained,vso that less, or-no further engraving of the band I5 will be required after it clears the sleeve.,

Incase the boat-tail bullet is used,v the prior practice'inraising the lands gradually from a cylindrical unried bore as a1; 4I of barrel 40.

(Figf)` in advance .of the neck .of the chamber may be followed, as at44 inFig. 7. Therelief 42 of the smooth bore for reception of our sleeve insuch case would `terminate short ofthe beginning of the vlands 43, this beginning M lof the. lands being advanced forward1y.:..in `the-,bore 1f/s section22 acting through sleeve I6 ensues. The

sleeve is also shaped to the plated riling Iby pressureof the rotating band, at the same time that rthe latter is engraved. When the bullet reaches the end'of the sleeve, additional engraving may take place directly onthe plated riiiing of the advanced bore section if there is lack of registry purposed or accidental of the shaped sleeve and advanced rifling. After firing, the case II, with lthe sleeve positively attached by the keying means 20 as; described is ejected from the gun, followed by loading, firing and ejection of similar sleeved rounds as described. rhe over-all effect is to relieve the gun barrel including forcing cone and riiied bore surface of exposure to the most severe conditions which normally contribute to erosion and plastic deformation, with the yresult that life of the gun is prolonged to a surprising extent. 4The chromium plating protects the advanced bore section ,from erosive effects and for itself is spared in large measure from mechanical effects set upk by they bullet. Actually, as the invention has been heretofore practiced, the bullet is largely or @Wholly engraved Ibefore coming into direct contact with the plated bore surface.

The function as described is the same with either the long or short sleeve and artillery type bullet. The function with the conventional hall cartridge of Fig. 3 will be understood without detailing, the sleeve simply protecting the smooth bore from erosion and deformation as will be understood, while the engraving of the bullet occursconventionally on the faced lands 45.

The forcing cone and relieved surfaces for accommodating the forward reduced end of the sleeve are not regarded as part of the chamber f of the gun, but the latter comprises only that enlarged part which accommodates the conventional `cartridge case and extending from the breech face of the gun tube ordinarily, to the forward end cf the neck portion 2. The chamber includes the shoulder 3:3 which receives thereagainst the shoulder I2 ofthe case II. The surfaces for- -wardly of the forcing cone are regarded as parts of thebore technically .so-called.

Results of numerous tests show that guns and ammunition provided and combined in accordancewith our invention to a large extent reduce lgun erosion and plastic deformation. The ammunition sleeves shield the breech bore surface from thermal, chemical and metallurgical, and

mechanical effects combined, and in ried guns o make it pcssiblewhen using raised rotating band valnmlunition to bring substantially pre-engraved bullets to the riing without any problem of orientation of the bullets in the barrel prior to firing. The erosion-resistant metal facing or lining on the advanced bore surface contributes much to the useful life of the guns, especially when provided in such thickness as to prevent heat alteration of the underlying steel and when afforded sleeve protection against initial stress and wear normally caused by engraving function and other forces.

In machine guns of the kind named herein there are recognized rates of fire (as, for instance, 800 shots per minute or more) which rapidly build up heat in the barrels, and it was a prior practice to limit the length of the bursts, and require intervals between bursts sufficient to permit dissipation of the heat transferred to the barrel. In water cooled guns this tendency to heat is reduced but not entirely eliminated, and the interior faces will pass the critical temperature under sustained automatic fire. Air cooled guns Will be more pronouncedly heated. yIn large caliber guns the frequency of re may be of the order of thirty shots per minute, more or less, according to the caliber, but due to the larger charges of propellant used, the barrels still accumulate heat and the allotropic form of the barrel material changes, as earlier stated herein. With our changeable sleeve, using a new one on each shot, there may be approximated a balance (between transfer of heat to the barrel, and its diusion in the barrel and dissipation by radiation, conduction and convection) in which a certain rate of re may be sustained indenitely, the sleeve shielding the barrel sufficiently, and its extraction removing heat before material transfer to the barrel, so that progressive dissipation of heat from the barrel may keep its mean temperature below a point of dangerous excess, especially adjacent the chamber in the part of the barrel ordinarily most subject to erosion. This will permit either continuous automatic fire as long as ammunition is available, or will permit much longer bursts at very high rates of machine gun fire; or more frequent ring of other types of guns.

Thus it will be seen that in the present invention guns and ammunition are provided and combined so as to prolong useful gun life, and that the objects hereinbefore noted together with many other thoroughly practical advantages are successfully achieved. The invention eliminates or minimizes many difiiculties encountered in repair and replacement of eroded guns and the associated problems of production and transportation.

It will be understood that while the use of chromium facings such as coatings, claddings, sleeves or plates covering the bore surface from and in advance of the ammunition sleeve is preferred, the facings may be dispensed with entirely, or other erosion resistant metal, composite metal, or alloy metal facings employed where practical.

It will be understood that it is within the scope of our invention to modify conventional guns, as by machining or electropolishing the bore surface so as to make room for the ammunition sleeve with or without the advanced protective facing, or to provide guns initially which are suitable to receive the ammunition sleeve with or without the advanced lining or facing of the bore.

Since many possible embodiments of our invention may be made, and as many changes may be made in the embodiments hereinbefore set 10 forth, it will be understood that the matter described herein is to be interpreted as illustrative and not invariable.

We claim:

1. A round of ammunition comprising in combination a case body shaped to t the chamber of a gun barrel, said case body shaped to hold a projectile, a projectile seated in said case body and projecting forwardly from the case body, said projectile having an enlarged part deformable by a gun barrel when fired therethrough, conventional primer and propellant means in the case body, and a sleeve on the case body extending forwardly thereof around said projectile beyond the enlarged deformable part, the sleeve shaped and proportioned to extend longitudinally within the bore of a gun barrel in which the round is loaded and in advance of said deformable part of the projectile so a's to lie over the bore area of principal erosive and corrosive tendency and means on said sleeve for positively retaining same on said case after the round is fired.

2. A round of ammunition comprising a case for conventional chambering in gun barrels, said case having an open forward end, a projectile seated in the forward end of said case and projecting therefrom, a thin sleeve in contact with said case and extending therefrom over at least a part of said projectile, and means for retaining said sleeve attached to said case after the round is red, said case being provided with a forwardly opening neck portion of reduced diameter and said projectile being of the banded artillery type and is seated partly within said neck portion and partly protruding therefrom, said projectile having a rotating band of diameter greater than the neck of the case, and said thin sleeve being of erosion resisting material and extending from said neck snugly over said band and around the largest diameter ofr said projectile.

3. In combination a round of ammunition and a gun barrel, said round comprising a case, a projectile seated in the forward end thereof, and a thin ductile, erosion resistant sleeve attached to and extending forwardly from said case over at least part of said projectile, said gun barrel comprising a rifled bore and a chamber for receiving said round, said chamber comprising a rified section corresponding to the sleeve portion of said round forwardly of said case, whereby said sleeve will be shaped by said rifling when the round is fired.

4. The invention as recited in claim 3 wherein the rified section of said chamber is enlarged to an extent equal to the thickness of said sleeve, the riing of said enlarged bore being aligned with the rifiing of the advanced bore, the width of the lands of said rifling of the enlarged bore being reduced correspondingly, wherein the rotating band of a projectile fired therein will be engraved on the inserted sleeve to match the riing of said advanced bore section.

5. A combined ammunition round and gun bore protecting device comprising a conventional ammunition round proportioned to fit a gun barrel having a conventional forcing cone, including a projectile and a sleeve of erosion resistant material fixed as a part of the round unit, said sleeve having a reduced forward part .extending forward around the projectile beyond the normal line of securement of the proiectile in the round and reduced so as to extend forwardly of the forcing cone of a barrel in which the round is inserted and to lie upon the bore surface of such barrel a substantial distance beyond the forcing -1rcone,' 'and 4means on' said sleeve' for'positively "',r'etain'ing same .in a gun barrel yafter the round Ais red, the projectile havingapartv of af cliaineter and material such as to tbe -engravable by rlfling of a rified bore barrel in which the round 6.` A combine-idv ammunitionround and gun bore irprote'ctingdevice comprising a conventional propellant charge, primer and container, a projectile 'extended Vforwardly therefrom and having an exf'posed part engravable by bore riding, and a sleeve of'erosion yresistant material xed aspart of the round unit and having apart extended forwardly around the projectile beyond said engravable-part 'so as to extend into 'the bore'of a barrel in which the'roundr is inserted and to lie upon the bore Asurface of such barrel'over a substantial distance beyond the chamber of such barrel and 'cooper- Lating locking means on said sleeveand retainer lfor/retaining said sleeve looked to sadcontainer ,-.after :the round is V fired, said means being operable'Y by gas'pressurey due to firing.

vHENRY B. ALLEN.

' NICOL I-L'SMITH.

e REFERENCES CITED The `following referencesv are of record in the le of this patent:

UNITED STATES PATENTS Number f Name Date Re. 18,252 Alden Nov.' 17,' 1931 270,616 "Taylor 'Jan. 16, 1883 452,601 Elliott May 19, 1891 A694,675 V$00121; Mar/4, 1902 694,895 Scott Mar. 4,' 1902 l868,938 Puff Oct. 22, 1907 1,021,685 Knaebel Mar. 26, 1912 1,355,422- Pedersen 11 Oct. 12, 192() 1,552,864 AMethln Sept. 8, 1925 1,886,218 Olin etal. Nov. l, 1932 1,944,883 Gerlich 'Jan. 30, 1934 2,112,144 Coupland Marf22, 1938

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