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The Effect of Crowning on Gun Barrel Individuality
The Effect of Crowning on Gun Barrel Individuality JOHN E. MURDOCK Criminalist, Contra Costa County S h e n f s Department, Criminalistics Laboratory, Martinez, California 9 4 5 5 3
Pajwpresented at the second annual conference of the Association of Firearm and Tool Mark Examiners, Chicago, Illinois, February 24th and 25th, 1970. Ths e$ect of crowning on gun barrel individuality is examined by comparing test Jirings from four button n~%d- 2 2 caliber n$e barrels. All barrels were testjred, recrowned and test Jired again. Three of the four barrels were recrowned a second time and test Jircd. A comparison of test bullets from each barrel with bullets from the same barrel following recrown revealed that although some changes in stria pattern were observed, suJ&nt agreement was noted to enable an identijication to be eJected.
Introduction A review of the firearms literature which concerns itself with explanations relating to the causes of gun barrel individuality reveals a scarcity of statements pertaining to the role of the nature of the rifling a t the muzzle. Several authors have, however, seen fit to comment on this role: Hatcher, Jury and Weller (1957), while doing experiments on barrels that were originally positioned adjacent to each other and finished together with their chambers placed in the same positions, stated "much of the individual characteristics of every barrel comes not from the operations proper performed on it, but rather from haphazard scratches; for instance, from chips in the reaming of the original bore, in the chambering of the barrel, or in the finishing and countersinking of the muzzle". Mathews (1962) commented that "in addition to the markings made by the rifling, marks may be made when the bullet strikes the forcing cone (if one is present) and by irregularities that exist a t the muzzle". Gunther (1930) made the observation that "conditions existing in the bore right at the muzzle may play a very important part in engraving the surface of the bullet, and this is especially true in the case of a lead bullet". We thus see that various writers have commented on the possibility that conditions which exist in the rifling at the muzzle of a gun barrel may influence its individuality. Although several authors have commented on this possibility, this writer is not aware of any published material relating to actual testing of the influence of the nature of the rifling a t the muzzle. This writer's attention was focused on this scarcity of information while evaluating the results obtained by Austin (1969) which involved his identification of bullets fired from consecutively rifled gun barrels. A question which came to mind was what influence the crowning operation would have upon the evaluation of projectiles fired from consecutively rifled gun barrels. If it is the crowning operation which imparts individuality to a gun barrel, the fact that they were consecutively rifled would be of no consequence. Crowning a gun barrel involves performing those mechanical operations upon the muzzle of a gun barrel which serve: (1) not only to protect the end of a bore from a bump which could damage the rifling a t this critical point, but also (2) results in the end of the bore being absolutely true and square, thus preventing the escape of a jet of powder gas from one side first resulting in the bullet being tipped as the base emerges (McFarland, 1965).
For the purpose of evaluating the effect ofcrowning on gun barrel individuality this writer obtained a -22 caliber rifle barrel from each of the following firms: Ithaca Gun Company, Inc.; Marlin Firearms Company; 0. F. Mossberg & Sons, Inc. ; and the Remington Firearms Company, Inc. The method of rifling employed by these firms in the fabrication of the barrels they provided is listed in Table 1. METHOD
TABLE 1 O F RIFLING EMPLOYED BY THOSE MANUFACTURERS WHO PROVIDED .22 CALIBER RIFLE BARRELS
Manufacturers Ithaca Gun Company, Inc. Ithaca, New York 14850 The Marlin Firearms Co. 79 Willow Street New Haven, Conn. 06502 0. F. Mossberg & Sons, Inc. 7 Grasso Avenue P.O. Box 497 North Haven, Conn. 06473 Remington Arms Company, Inc. Ilion, New York 13357
Method of Manufacture The barrel was drilled, reamed, ball sized and button rifled. The barrel was rifled with a carbide button.
The barrel was rifled by pulling a carbide button through the barrel.
The barrel was rifled using a swage process.
Experimental Each of the four barrels was chambered for the -22 caliber long rifle cartridge and a standard crown was formed a t the muzzle (Figs. 1 and 2) using a crowning bit fabricated from bar stock on hand in the local gunshop whose co-owner performed all of the metal working operations described herein. Each barrel was fitted to a suitably altered bolt action taken from a Mossberg Model 42M(b), -22 caliber rifle. Following the initial crown and after each recrown, the barrels were cleaned with an oil cotton mop attached to a "Parker-Hale" brand cleaning rod. This type of cleaning rod was utilized because the coating on the rod reduces the chances of any alteration of the rifling due to mechanical abrasion. Each barrel was then test fired three times using -22 caliber Remington Peters Standard Velocity long rifle ammunition. One box of 50 rounds proved more than enough for test purposes. All test bullets were fired into a vertical tank filled with water and collected undamaged. Following the first three test firings, each barrel was recrowned using the same crowning bit as employed in the original crown. Each crowning and subsequent recrown was checked by examining the crowned edge with a 5 x magnifying glass. Any flaws revealed were corrected with the crowning bit prior to test firing. Following the first recrown, a fourth, fifth and sixth test bullet was fired and collected from each barrel. All barrels except the Marlin were recrowned a second time and a seventh, eighth and ninth bullet was test fired and collected from each. I t should be noted that during each recrown, the barrels were shortened by approximately three millimeters. Examination Results and General Discussion A comparison was effected between test bullets from each barrel and test bullets from the same barrel following recrown utilizing a Martin-Held bullet comparison microscope. This comparison revealed that although some changes in striation pattern were observed on test bullets which were separated by a recrown, sufficient agreement was noted to enable an identification to be effected. This ability to make an identification in spite of the fact that the barrel has been recrowned is interesting from several standpoints, the first of which is that it increases the significance of experiments made where test-fired bullets from consecutively rifled barrels were examined without first considering the possible
Fig. 1.
The crowning operation
Fig. 2. The crowning operation
Fig. 3.
Ithaca rifling
Fig. 4. Marlin rifling
Fig. 5.
Mossberg rifling
Fig. 6. Remington rifling
effects of the crowning operation. These results are also interesting because an examination of the rifling present in the four barrels revealed the presence of rather coarse reaming marks (Figs. 3-6) on both the land and groove surfaces in all of the barrels except the one manufactured by the Remington Arms Company. An examination of the rifling of the Remington barrel failed to reveal the presence of any reaming marks on either the lands or grooves. An intuitive approach to the problem suggests that if crowning were to have any effect on individuality, it should be greatest in the barrels which show rather coarse reaming marks at the muzzle since it is this rifling which the fired bullet contactsjust prior to leaving the barrel. Since the recrowning operation shortens the barrel, the original reaming marks which the bullet contacted at the muzzle are removed and a new set is presented. Since the Remington barrel does not have any apparent reaming marks, one would not expect that due to a recrown bullets test fired through it would exhibit sufficient changes in the striation pattern to prevent an identification to be effected. The presence of reaming marks in the other three barrels suggests that a recrown would affect individuality perhaps to the point of not being able to effect an identification. Since experimental results, as discussed above, have revealed the minimal effect of a properly performed crown on gun barrel individuality, statements relating to the role of the various methods of rifling upon individuality now have added meaning.
Acknowledgments The author wishes to thank the four firearms manufacturers who provided the rifle barrels, and Forrest Motter, the gunsmith who performed the mechanical operations on them. -
References AUSTIN,P. F., 1969. Paper presented at the Fifth International Meeting of Forensic Sciences, University of Toronto, Toronto, Ontario, Canada. GUNTHER, C. O., 1930. Mechanical Engineering, 52, 1069. HATCHER, J. S., JURY,F. J., and WELLER,J., 1957, Firearms Investigation Identification and Evidence, p. 371, The Stackpole Co., Harrisburg, Penn., U.S.A. MACFARLAND, H. E., 1965, Introduction to Modern Gunsmithing, p. 200. The Stackpole Co., Harrisburg, Penn., U.S.A. MATHEWS, J. H., 1962. Firearms Identijcation, p. 11, The University of Wisconsin Press, Madison, Wisconsin, U.S.A.
Pajwpresented at the second annual conference of the Association of Firearm and Tool Mark Examiners, Chicago, Illinois, February 24th and 25th, 1970. Ths e$ect of crowning on gun barrel individuality is examined by comparing test Jirings from four button n~%d- 2 2 caliber n$e barrels. All barrels were testjred, recrowned and test Jired again. Three of the four barrels were recrowned a second time and test Jircd. A comparison of test bullets from each barrel with bullets from the same barrel following recrown revealed that although some changes in stria pattern were observed, suJ&nt agreement was noted to enable an identijication to be eJected.
Introduction A review of the firearms literature which concerns itself with explanations relating to the causes of gun barrel individuality reveals a scarcity of statements pertaining to the role of the nature of the rifling a t the muzzle. Several authors have, however, seen fit to comment on this role: Hatcher, Jury and Weller (1957), while doing experiments on barrels that were originally positioned adjacent to each other and finished together with their chambers placed in the same positions, stated "much of the individual characteristics of every barrel comes not from the operations proper performed on it, but rather from haphazard scratches; for instance, from chips in the reaming of the original bore, in the chambering of the barrel, or in the finishing and countersinking of the muzzle". Mathews (1962) commented that "in addition to the markings made by the rifling, marks may be made when the bullet strikes the forcing cone (if one is present) and by irregularities that exist a t the muzzle". Gunther (1930) made the observation that "conditions existing in the bore right at the muzzle may play a very important part in engraving the surface of the bullet, and this is especially true in the case of a lead bullet". We thus see that various writers have commented on the possibility that conditions which exist in the rifling at the muzzle of a gun barrel may influence its individuality. Although several authors have commented on this possibility, this writer is not aware of any published material relating to actual testing of the influence of the nature of the rifling a t the muzzle. This writer's attention was focused on this scarcity of information while evaluating the results obtained by Austin (1969) which involved his identification of bullets fired from consecutively rifled gun barrels. A question which came to mind was what influence the crowning operation would have upon the evaluation of projectiles fired from consecutively rifled gun barrels. If it is the crowning operation which imparts individuality to a gun barrel, the fact that they were consecutively rifled would be of no consequence. Crowning a gun barrel involves performing those mechanical operations upon the muzzle of a gun barrel which serve: (1) not only to protect the end of a bore from a bump which could damage the rifling a t this critical point, but also (2) results in the end of the bore being absolutely true and square, thus preventing the escape of a jet of powder gas from one side first resulting in the bullet being tipped as the base emerges (McFarland, 1965).
For the purpose of evaluating the effect ofcrowning on gun barrel individuality this writer obtained a -22 caliber rifle barrel from each of the following firms: Ithaca Gun Company, Inc.; Marlin Firearms Company; 0. F. Mossberg & Sons, Inc. ; and the Remington Firearms Company, Inc. The method of rifling employed by these firms in the fabrication of the barrels they provided is listed in Table 1. METHOD
TABLE 1 O F RIFLING EMPLOYED BY THOSE MANUFACTURERS WHO PROVIDED .22 CALIBER RIFLE BARRELS
Manufacturers Ithaca Gun Company, Inc. Ithaca, New York 14850 The Marlin Firearms Co. 79 Willow Street New Haven, Conn. 06502 0. F. Mossberg & Sons, Inc. 7 Grasso Avenue P.O. Box 497 North Haven, Conn. 06473 Remington Arms Company, Inc. Ilion, New York 13357
Method of Manufacture The barrel was drilled, reamed, ball sized and button rifled. The barrel was rifled with a carbide button.
The barrel was rifled by pulling a carbide button through the barrel.
The barrel was rifled using a swage process.
Experimental Each of the four barrels was chambered for the -22 caliber long rifle cartridge and a standard crown was formed a t the muzzle (Figs. 1 and 2) using a crowning bit fabricated from bar stock on hand in the local gunshop whose co-owner performed all of the metal working operations described herein. Each barrel was fitted to a suitably altered bolt action taken from a Mossberg Model 42M(b), -22 caliber rifle. Following the initial crown and after each recrown, the barrels were cleaned with an oil cotton mop attached to a "Parker-Hale" brand cleaning rod. This type of cleaning rod was utilized because the coating on the rod reduces the chances of any alteration of the rifling due to mechanical abrasion. Each barrel was then test fired three times using -22 caliber Remington Peters Standard Velocity long rifle ammunition. One box of 50 rounds proved more than enough for test purposes. All test bullets were fired into a vertical tank filled with water and collected undamaged. Following the first three test firings, each barrel was recrowned using the same crowning bit as employed in the original crown. Each crowning and subsequent recrown was checked by examining the crowned edge with a 5 x magnifying glass. Any flaws revealed were corrected with the crowning bit prior to test firing. Following the first recrown, a fourth, fifth and sixth test bullet was fired and collected from each barrel. All barrels except the Marlin were recrowned a second time and a seventh, eighth and ninth bullet was test fired and collected from each. I t should be noted that during each recrown, the barrels were shortened by approximately three millimeters. Examination Results and General Discussion A comparison was effected between test bullets from each barrel and test bullets from the same barrel following recrown utilizing a Martin-Held bullet comparison microscope. This comparison revealed that although some changes in striation pattern were observed on test bullets which were separated by a recrown, sufficient agreement was noted to enable an identification to be effected. This ability to make an identification in spite of the fact that the barrel has been recrowned is interesting from several standpoints, the first of which is that it increases the significance of experiments made where test-fired bullets from consecutively rifled barrels were examined without first considering the possible
Fig. 1.
The crowning operation
Fig. 2. The crowning operation
Fig. 3.
Ithaca rifling
Fig. 4. Marlin rifling
Fig. 5.
Mossberg rifling
Fig. 6. Remington rifling
effects of the crowning operation. These results are also interesting because an examination of the rifling present in the four barrels revealed the presence of rather coarse reaming marks (Figs. 3-6) on both the land and groove surfaces in all of the barrels except the one manufactured by the Remington Arms Company. An examination of the rifling of the Remington barrel failed to reveal the presence of any reaming marks on either the lands or grooves. An intuitive approach to the problem suggests that if crowning were to have any effect on individuality, it should be greatest in the barrels which show rather coarse reaming marks at the muzzle since it is this rifling which the fired bullet contactsjust prior to leaving the barrel. Since the recrowning operation shortens the barrel, the original reaming marks which the bullet contacted at the muzzle are removed and a new set is presented. Since the Remington barrel does not have any apparent reaming marks, one would not expect that due to a recrown bullets test fired through it would exhibit sufficient changes in the striation pattern to prevent an identification to be effected. The presence of reaming marks in the other three barrels suggests that a recrown would affect individuality perhaps to the point of not being able to effect an identification. Since experimental results, as discussed above, have revealed the minimal effect of a properly performed crown on gun barrel individuality, statements relating to the role of the various methods of rifling upon individuality now have added meaning.
Acknowledgments The author wishes to thank the four firearms manufacturers who provided the rifle barrels, and Forrest Motter, the gunsmith who performed the mechanical operations on them. -
References AUSTIN,P. F., 1969. Paper presented at the Fifth International Meeting of Forensic Sciences, University of Toronto, Toronto, Ontario, Canada. GUNTHER, C. O., 1930. Mechanical Engineering, 52, 1069. HATCHER, J. S., JURY,F. J., and WELLER,J., 1957, Firearms Investigation Identification and Evidence, p. 371, The Stackpole Co., Harrisburg, Penn., U.S.A. MACFARLAND, H. E., 1965, Introduction to Modern Gunsmithing, p. 200. The Stackpole Co., Harrisburg, Penn., U.S.A. MATHEWS, J. H., 1962. Firearms Identijcation, p. 11, The University of Wisconsin Press, Madison, Wisconsin, U.S.A.