What would cause a blister like bulge on a rifle cartridge below neck when firing?

Answer 1

When a round is chambered for firing, it should be fully sealed evenly all around. When the propellant ignites, the gasses push out in all directions, forcing the sides of the catridge hard up against the chamber - the rest of the gasses then propell the round up the barrel.

Bercause the chamber is the exact same size of the catridge the propelling gasses should not be able to exert more force in one particular area.

If there is uneven deformation of the fired round the causes are either 1) a damaged chamber or 2) build up of debris/oil/grease or 3) the round is not fully chambered. Oil or grease in the chamber will alter the chamber profile, so the gasses cannot expand the round evenly. Likewise, debris/oil/grease in the chamber locking mechanism may be enough to just prevent the chamber fully locking shut but still fire the round

There is an outside chance it is an ammuntion fault, but only a very small one for the previous reasons explained.

From my experience, it is most likely a hard deposite in the chamber, the area of the neck of the round, preventing the round being fully chambered. The part of the round not in contact with this dirt will not be in contact with the chamber wall and will be the part deforming, as it expands on firing.

Your first step should be to inspect the chamber and breech to look for obvious damage or dirt. Take the rifle to a gun smith for testing, he should have a machined gauge to test the tolerances inside the chamber. I wouldn't fire the rifle anymore until you have confirmation the chamber isn't damaged - you don't want the gasses of a fired round coming out of anywhere but the end the barrel!

Ex. Navy Ordnance Engineer