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I was given back a 9mm upper form a AR 9 rifle I built and gave to the son of a old friend for helping me move a few years back, He went with a different upper and gave me the older one back. He had brought the rifle back to me to look at a few months ago to see why it would not eject the cases any more, turned out the ejector on the mag adapter was a little bent and would not touch the case as the bolt came back so I tweaked it with a pair of pliers and it was fine, He told me his sister was shooting it and a round blew up when she shot it and I did not think any thing about it.
It had evidently been a squib load that only went about 7" down the bore and had enough pressure to function the action and load another round which she fired as well, the air pressure between the two bullets must have pushed the stuck bullet out along with the second but as the case was coming out there must have still been high enough it burst the case. This is a 16" heavy barrel, it is .840" in diameter on both sides of the bulge.

OK now that I got the story out of the way do you think it is safe to shoot a ringed barrel? I mic'd the out side of the barrel at the bulge and it is about .012" bigger the rest of the barrel. I am thinking about cutting it off at 5 1/2"s and having it crowned and re-threaded and use it for a pistol build. I also thought about using the rest of the barrel to make a adapter to shoot 9mm out of a single shot shotgun, there is a good 8" in front of the bulge.

Casey
 

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I'm guessing it is safe to shoot, in that it won't burst, but I can see it causing all sorts of other problems. Ringing occurs when the second bullet hits the first, so the bullets briefly compress together and get fatter as a result, and bulge the barrel.

With a 0.012" bulge, that is more than the difference between the land and the groove, so the rifling will loose contact. Plus, you will get gas flow around the bullet, causing throat-type erosion in the barrel. If you lose contact with the bullet, that is like having too much freebore at the throat, so the bullet can misalign and cause keyholing .... just a hot mess. Cutting it is probably the only way to salvage anything.
 

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Scrap it and buy a BCA. Err on the side of safety and besides, it sounds like the likelihood of unforeseen problems mean you don’t want to chase that rabbit down the hole.
 

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If I'm reading this right... .012" is only .006" per side of the bore. Most rifling, I believe, is more on the order of .020" deep or better per side.
There should still be plenty of rifling contact with the bullet.
Normally, I'd be looking to replace it, but this is 9mm and a plinker.

A good friend of mine ran a select-fire HK MP5 and had a squib run through during FA firing.
It left a small bulge just aft of the front sight ring. At the time he showed me this he had run an additional 18,000rds through and it was running perfect and still a tack driver.

He had purchased a new barrel and it still hasn't been needed.
 

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I'd shoot it and see how it prints. If you cut and recrown will the alphabet gang make reservations to the club fed? Also are you doing the work if not buying a new barrel might be cheaper?

Say can the lower take 10mm? If so I'd go that way. Or .40S&W being that 9mms are hard to locate and commanding silly prices.
 
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