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Old 08-07-2006, 03:14   #274
RtBrane
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Join Date: Aug 2006
Location: Kalifornia
Posts: 19
G27 bullet setback when chambering

Normally, I load my G27 (40sw) by chambering a round from a fully loaded mag, then struggle to top off the mag. Wait a minuite, I've got an empty mag, I'll put a single round in that for loading the chamber. Yeah, right. Jammed against the barrel ramp. Start over, same problem. Tried slingshot and slide release, same result. Not everytime, but better than 50% jammed. (Through several classes, this gun has never malfunctioned.) Tried several different brands, hollowpoint and flatpoint. All will jam. Smacking the bottom of the mag will cause the round to finish chambering.
After this, I noticed the rounds didn't look right. Compared them to fresh ammo, realized all had some setback.
Hmmm, time to bring out the calipers and run a small test.
Jammed produced setback from 0.015" to 0.065"
Non-jam loading also produced setback, as much as 0.019"
However, I also measured bullets moving out after chambering. 0.001 to .004" Appearently, the slide velocity is rather high.

Obviously, the chambering dynamics are different between hand cycling and firing, since I've run mags dry many times without any jamming on the last round.
However, the bullet displacement that this little test indicates, brings me to the tentative conclusion that this may be the causitive factor in some of the case ruptures, and chamber failures (KBooms). Especially the puzzling factory ammo KB's.
It may also explain the accuracy complaints about the .40sw, since varying the powder space in the case will effect pressure which changes bullet placement at the target.

BTW, for those interested, the minimum chamber wall thickness I found was 0.080" (RH side). I measured the chamber walls of a Colt Pocket9 (only 9mm on hand), minimum was 0.074"

I think I'll take calipers with me next time I shoot, and check rounds that have been dynamically loaded.

Will
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