What caused this bulge in the 9mm Luger shell case?

[martyd]

9mm Luger Case

I refer to this as a pregnant shell case.

There is a bulge at the base of the case which is quite noticeable.

I appears to me that the case is not chambered all the way when firing.
Which could happen with a full automatic firearm.
OR
There is a defect in the barrel chamber.

I see this fairly often on range brass.

Sorry I couldn't get a good pic from the side.

Any idea what causes this?

What caused this bulge in the 9mm Luger shell case?

 

[Suck My Glock]

You are absolutely correct that this is from a partially unsupported chamber, specifically from near the feed ramp cut, as can typically be found in 1911-style linked barrel locking systems. This can get even more distinct and noticeable if someone has tried to do a feed reliability job on the feed ramp cut area of the barrel/chamber and has actually relieved (removed) even more material away from the barrel.

You can see here where a ramped chamber has a more fully supported rear chamber area than a traditional non-ramped barrel.

 

[Boriqua]

I would see that on some of my 10mm shot through a Glock 20. I landed up putting in a new aftermarket barrel because while a press can most times get that bulge out I cant imagine that stretching and compressing the brass would aid longevity.

I believe from old threads I have read across the web that some of the early glocks had issues with the 40 not being well supported.

Is this your brass? if so what gun?

 

[Flash]

I've never seen that on a 9MM case. Wonder what gun it was shot in.

 

[knockonit]

See it all the time on 9mm and .45, not an issue, i still reload them, if case is in decent shape,

 

[omegaman]

pressure caused it! lmao Anyhow if your reloading it use a lee factory crimp dye in the last station it will size the lower part of the case.

 

[delta6]

Well, I hate to disagree with everyone, but I do not think that case is bulged at all. That crease, which indented the case, makes it look like the web area is bulged. That crease appears to be an ejection crease. Very common on many subguns and all SIG 550 rifles.
Measure the case mouth and below the crease and you'll have your answer.

 

[xerts1911]

What delta6 said, measure the case mouth and give us your findings please

 

[Old Jeff H]

To me it looks like a bulge, like the examples in these threads:

http://forums.thecmp.org/showthread.php?t=153523

https://www.nationalgunforum.com/threads/glock-bugle-9mm-taper-lee-bugle-buster.190378/

(You can see it better if you click on the OP's full size image)

 

[delta6]

You can also get that crease on a single stage loading press during the sizing operation. Blow up the picture and look at the case mouth. Odd?
Measurements will tell us a lot.

 

[YNOTAZ]

I agree, looks like a sub-gun fired and the brass resized. If the camera isn't lying it looks like the sizing die didn't go all the way to the base. Measuring the case
at the mouth, belled for reloading,
center, sized correctly,
base, not resized.