Cartridge fired before fully chambered?

[Spook]

Has anyone ever had a cartridge fire while not completely in the chamber? Had one t'day ....blew out the side at the base of the case, bent the extractor... case ended up looking like a very short belted magnum on the side that wasnt blown off. The rifle fired 25 or so prior to being disabled without a hitch. Another WTF?

[pug]

Spook is this an AR-15? If so were you shooting high velocity rounds out of an upper that has had the gas port opened up to cycle subsonic?

[Spook]

It is an AR, I was shooting high vels, but I have no idea on how big Model 1 makes the port.....sounds like you know something I dont.

[pug]

Actually know very little and I assume they are still shipping their uppers ported for high velocity rounds. However if you open up a carbine positioned gas port so it will cycle subsonic ammo and do not install a gas valve you really should not shoot high velocity ammo. The reason is shooting high pressure rounds dump to much gas too fast into the carrier causing the bolt to unlock violently and start the extraction too soon. If the case extracts and is unsupported while the pressure is high you may blow a case just like you described. I am sure you checked the bore for obstructions but mention it because firing a round after a squib will blow a case and extractor also.