7.65 cases bulging at the base

[wolfee]

After a few thousand rounds my 83 is getting VERY smooth.  Apparently too smooth.  Now the action opens so easily that it seems like it might be beginning it's reward motion before the pressure has dropped.  Most cases from most brands of ammo now have a slight bulge on one side just in front of the case head.  Is this dangerous?  I asked CZ and they said normal.  I have a hard time believing this.

[steve98661]

I have seen the same on my CZ82 with high power ammo. Same thing, only on one side.

I did a lot of measurements on the ammo with calipers and found that the high power ammo with smaller diameter of casings is most likely to bulge ever so slightly. The bulges are not any larger than the completely circular bulges I get on other 9x18 pistols which is supposedly normal case expansion.

I found that using normal power ammo like S&B and making sure the front part of the chamber in the barrel area, the rim, is very clean help with this.

Look on google for 'unsupported chamber' information on glocks to find out more about this type of issue.

If you have your pistol field stripped, drop a round in the chamber and look at the bottom of the round, the cutaway portion of the ramp.  You will see that this is the area a one side bulge is created on, mostly likely.

Fiocchi ammo is 220ft lbs/1200 fps in 32 acp and corbon is up there too.  If you are using ammo like this, the "web" area of the shell is more likely to bulge slightly. Supposedly, most manufacturers strengthen the "web" area where you saw your bulge, probably.

[Sentient12]

Would you define the "web area" of a casing, and could you provide a link to a resource using that term?

[steve98661]

Unsupported chamber: With a round fully inserted, a portion of the casing next to the extractor gap is exposed. When the round is ignited, this portion of the casing can expand because there is no metal there from the chamber to support it and prevent expansion.

Web: The portion of the casing closest to the extractor gap. Manufacturers of ammo supposedly make this portion  of the casing heavier or thicker to prevent bursting of the casing.

Why is a portion of the casing exposed, in an unsupported fashion? The feed ramp is smooth and to allow a smooth transition of the round into the portion of the barrel called the chamber, a small portion of the chamber edge is ground away and smoothed also resulting in the unsupported portion of the chamber/round.

Never, never grind away at your feed ramp or the chamber edge!

Many, many semi auto pistols have a small portion of 'unsupported chamber' area straight from the factory.

What is a tell tale sign of problems with an unsupported chamber? Expansion of casings closer to the primer end of the casing, next to the extractor gap at the '6 oclock' position ON ONE AREA ONLY OF THE CIRCUMFERENCE ONLY. This doesn't include the normal expansion of casings all of the way around the casing.

It is common for almost all semi auto pistols to have some portion of the round unexposed from what I've read.. Guns that feed well will probably have a portion of the chamber ground away next to the feed ramp, making a portion of the casing unsupported. If it was done by the factory its safe. If it was done by someone else, don't trust it.

Glock and other manufacturers have focused on higher calibers such as 40 and above for problems, but it can happen with any caliber. Experts say that polygonal bores, lead (not copper coated) bullets, and excessive pressure rounds contribute to unsupported incidents that result in bursting of the casing.  I am by no means badmouthing Glock!

Here's a link to some info. Annotation number 4 and 5 show the casing in the chamber and the portion of the casing exposed (unsupported by chamber wall).

www.thegunzone.com/glock/kb-notes.html

My CZ82 has been tuned by the armory, I suspect. It feeds like a champ, but using higher power ammo on it results in casing bulges in one spot. Part of the chamber was ground away at the factory or armory to improve feeding. I can see the striations from a circular metal brush on the feed ramp.

Certain rounds are longer from the lip of the casing to the extractor gap. These rounds expose even more of the casing which could result in bulging on ignition.

I don't use 120g old school silver bear, nor custom loads sold on the web. Both have resulted in 6 oclock bulges. 94g silver bear, 95 S&B, and others work great, no bulges for me on my 9x18 CZ82.

It is harder for smaller rounds to get unsupported symptoms  in my opinion because the ratio of the pressure on ignition of the round to the thickness of the wall is better.

If you get any symptoms as mentioned above , take your pistol to a gunsmith immediately!  If you have a catastrophic failure involving a bursted case, you could really get hurt!

[Sentient12]

Thanks.  The term "web" was what I asked about - it's not a standard term in any of my reading.

[EIGHTYDUECE]

Your pistol may be ready for a new recoil spring.  straight blowback pistol recoil springs wear out alittle faster than the delayed blowback pistols.  I change mine out after 1500 rounds or so.