Two weeks ago I posted on some experiments with bullet setback, supposedly a common cause of guns, especially auto pistols, blowing up. Bullet setback occurs, according to the theory, when repeated loading of a single round (like the top round in a magazine) causes the bullet to be forced back into the case by the feed ramp, or the recoil from firing of the first rounds in a magazine causes the bullets of later rounds to be forced back. In either situation, the bullet being seated deeper in the case than normal will, supposedly, cause extremely high pressures and will very often cause the gun to blow up.
At that time, I reported experiments with 9mm Parabellum, which basically showed that there was indeed an increase in pressure, as best I could judge without pressure testing equipment, but that I did not think the pressure rise was enough to blow up a gun.
This week, I repeated the experiment with two other calibers, .40 S&W and .45 ACP. I used factory loads throughout, to reduce/eliminate the possibility of a double charge and because the variety of handloads is almost infinite. As with the 9mm, I used two different brands of ammunition in each caliber.
This is what I found.
.40 S&W
The cartridge overall length (COL) of both brands (Winchester and Independence) was 1.123"; two cartridges of each brand were fired from the box as controls.
Using a vise, I pressed the bullet into the case to obtain a COL of 1.097" for two cartridges of each brand. I repeated this to obtain a COL of 1.047" for two cartridges. For a third pair of cartridges, I pushed the bullets in as far as they would go. This was 1.005" for one brand, and .958 for the other. The bullets were seated on the powder charge and the difference apparently was in the amount of powder. I fired all the rounds in a S&W .40 Sigma.
The results:
The control cases showed normal primers and no head expansion.
The cases from the 1.097" rounds showed normal primers and no head expansion.
The cases from the 1.047" rounds showed flattened primers and some extrusion into the firing pin hole, but no head expansion.
The cases in which the bullet was seated as far as it could go showed primer extrusion into the firing pin hole and head expansion of .003"-005". That is a sign of significant pressure rise, but not, IMHO, enough to blow up a gun or even burst a case given normal support.
Again, while deep seating a bullet can cause a pressure rise, I doubt that bullet setback alone can cause enough of a pressure rise to result in blowing up a gun, at least with factory loads. I might also point out that the extreme bullet setback of the latter situation is impossible to conceive as happening accidentally. It took hard, deliberate pressure on the vise handle to get the bullets that deep and the cases were bulged.
.45 ACP
Again two brands were used, GI WCC and Hungarian Lake City, both GI spec loads. COL of both was 1.253". The first setback was to a COL of 1.180"; the second to 1.167". As in the .40 experiment, the bullets of the last four rounds were pressed in to seat on the powder with COL's of 1.044" and 1.074" respectively. All rounds were to be fired in a Norinco pistol.
The results were somewhat different. The control rounds and the first two setback rounds showed no sign of high pressure; primers were normal and case heads showed no expansion at all. I was unable to test the rounds with the bullets seated all the way in because the seating bulged the cases to the extent that they would not chamber.
So, again, the results are not really conclusive. I have not proved, absolutely, beyond any doubt, that bullet setback will not cause a blown gun. But I believe that I have proven that bullet setback will not ALWAYS cause a blowup, and it is my opinion that automatically placing the blame for high pressure on bullet setback may not be valid. I think it might be more productive, at least when dealing with reloads, to consider the more usual causes of high pressure, like an excessive charge or improper loading.
Jim
At that time, I reported experiments with 9mm Parabellum, which basically showed that there was indeed an increase in pressure, as best I could judge without pressure testing equipment, but that I did not think the pressure rise was enough to blow up a gun.
This week, I repeated the experiment with two other calibers, .40 S&W and .45 ACP. I used factory loads throughout, to reduce/eliminate the possibility of a double charge and because the variety of handloads is almost infinite. As with the 9mm, I used two different brands of ammunition in each caliber.
This is what I found.
.40 S&W
The cartridge overall length (COL) of both brands (Winchester and Independence) was 1.123"; two cartridges of each brand were fired from the box as controls.
Using a vise, I pressed the bullet into the case to obtain a COL of 1.097" for two cartridges of each brand. I repeated this to obtain a COL of 1.047" for two cartridges. For a third pair of cartridges, I pushed the bullets in as far as they would go. This was 1.005" for one brand, and .958 for the other. The bullets were seated on the powder charge and the difference apparently was in the amount of powder. I fired all the rounds in a S&W .40 Sigma.
The results:
The control cases showed normal primers and no head expansion.
The cases from the 1.097" rounds showed normal primers and no head expansion.
The cases from the 1.047" rounds showed flattened primers and some extrusion into the firing pin hole, but no head expansion.
The cases in which the bullet was seated as far as it could go showed primer extrusion into the firing pin hole and head expansion of .003"-005". That is a sign of significant pressure rise, but not, IMHO, enough to blow up a gun or even burst a case given normal support.
Again, while deep seating a bullet can cause a pressure rise, I doubt that bullet setback alone can cause enough of a pressure rise to result in blowing up a gun, at least with factory loads. I might also point out that the extreme bullet setback of the latter situation is impossible to conceive as happening accidentally. It took hard, deliberate pressure on the vise handle to get the bullets that deep and the cases were bulged.
.45 ACP
Again two brands were used, GI WCC and Hungarian Lake City, both GI spec loads. COL of both was 1.253". The first setback was to a COL of 1.180"; the second to 1.167". As in the .40 experiment, the bullets of the last four rounds were pressed in to seat on the powder with COL's of 1.044" and 1.074" respectively. All rounds were to be fired in a Norinco pistol.
The results were somewhat different. The control rounds and the first two setback rounds showed no sign of high pressure; primers were normal and case heads showed no expansion at all. I was unable to test the rounds with the bullets seated all the way in because the seating bulged the cases to the extent that they would not chamber.
So, again, the results are not really conclusive. I have not proved, absolutely, beyond any doubt, that bullet setback will not cause a blown gun. But I believe that I have proven that bullet setback will not ALWAYS cause a blowup, and it is my opinion that automatically placing the blame for high pressure on bullet setback may not be valid. I think it might be more productive, at least when dealing with reloads, to consider the more usual causes of high pressure, like an excessive charge or improper loading.
Jim