I just got a P320 3.9" and I'm getting some cases with a bulge at the base. These are reloads of a tried and true load. 40S&W- 180 FN over 5.6g of WSF. I've fired 1000's of these in many different pistols. What gives? It's mixed brass but primarily Starline both brass and nickle cases. What should I look for?

It’s caused when rounds are fired through a Glock that doesn’t have a fully supported chamber. It’s referred to as “Glock Bulge”. Search for the Lee Bulge Buster on Youtube. That tool is really like full-length sizing the brass in reverse and will fix 99% of the cases that have a bulge. I’d post a video, but I’m on my iPad and it’s a PITA...

Went into my office and got on the PC. This video explains the bulge issue, but what I'd add to this guy's process/routine would be to test each piece of brass in a Case Gage...I use L.E. Wilson Case Gages.

Thanks, I'm aware of the Glock bulge but didn't expect to find it in a Sig. A side by side with my CZ75B in .40. Same ammo, same session at the range and zero problems with the CZ. When I plunk tested the same rounds in each gun all was well. The difference is the taper around the feed ramp in the Sig. It is enough to allow the "Sig" bulge. I'm disappointed in Sig. The CZ is a straight wall to the end of the ramp and bullet case.
That relegates the Sig to defensive only with factory loads and one-use brass. I know I can resize the brass but my faith in the pistol is diminished.

Ooops...sorry sir. I obviously didn’t mean to insult your intelligence. I kinda didn’t connect the dots about this happening in your Sig. Durp!

No offense taken. I'm just venting about my P320.

I plan to load 50 rds with new Starline brass and same everythng else:180gr/5.6 WSF/COL and check all that brass. Maybe used brass loses some strength in the case wall. I'll report back.

I have started seeing this more after I have reloaded a piece of brass numerous times. It is usually right before the case starts mushrooming towards the headcase but still on the side of the brass. That is the time I throw it into the junk pile of brass. I had pictures of the cases posted on Instagram. I'll try finding them. Sorry I can't find those pictures.

I'm curious...is this happening with factory loads as well, or just that specific handload? You've got me concerned....I've been holding off buying a Glock in 10mm for years (I don't need an unsupported chamber destroying expensive brass) and waiting for Sig to come out with a P320 in 10mm for this very reason. If the .40 P320s are bulging brass, that doesn't leave much hope that a future 10mm will be any better.

I've personally seen 9mm Glocks bulge brass...I've never had that happen with a P320. But I've never shot a P320 in .40, either.

For Glock’s, you can always get an aftermarket barrel which provides more case support and standard rifling in case you want to shoot lead bullets to save money.

I finally got to the range. All is good news. The new Starline cases showed no bulges and functioned perfectly. Not a single failure of any kind and accuracy was spot on through 70 rds. I'm a happy camper. I'll be segregating my brass to keep only the newest for the Sig. My CZ 75B forties will get the older brass and I'll monitor that brass too.