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Old October 29, 2012, 10:42 AM   #38
carguychris
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Join Date: October 20, 2007
Location: Richardson, TX
Posts: 7,523
Quote:
I believe this setback issue is more of a 40S&W thing more so than other calibers... The 40, if I remember correctly, has a rather dramatic pressure increase for a given amount of setback.
Setback is potentially worse in .40S&W than other commonplace auto pistol calibers because (a) the case volume is small and (b) the empty portion of the case is relatively wide yet short, so a small amount of bullet movement will cause a proportionally larger change in volume.

According to my math, the .45GAP potentially also has this problem, although it operates at substantially lower pressure than .40S&W. I have not heard about this round having a higher-than-normal percentage of kB!s than other cartridges, but then again, I've not had a chance to ask one of the approximately 63 people in the USA who carry a G37/38/39 and aren't cops.

It should be emphasized, however, that ANY auto pistol cartridge that headspaces from the case mouth can have this problem. The potential exceptions are semi-rimless rounds that headspace from the rim, such as .32ACP, .38ACP (not .380!), and .38 Super. The case of a semi-rimless round can be crimped into a cannelure in the bullet like a revolver round; in fact, I inspected some .32ACP Fiocchi 73gr FMJ ammo I have, and it has cannelured bullets and crimped cases.
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