A5 rear bolt o-ring

I've had 2 A5's for about 4 years and other than an a very occasional ball break have been problem free until now. Both are doing pretty much the same thing. Today I went through 4 rear bolt o-rings and shot maybe 300 balls. Sometimes it's the first shot with a new o-ring and the gun is dead and others it may be 2 hoppers full of paint. What's happening is the real bolt o-ring is popping off the rear bolt and then getting mangled by the power tube and bolt when the gun is fired. You never know when it will happen. The guns are polished inside and have been for several years. The replacement o-rings are new. There's no debris in the guns. I've taken them both totally apart, cleaned/oiled everything including the valve parts. there's nothing in the guns to catch the o-rings. I've used the original o-rings for years until now. They seem to pop out of place as the bolt travels towards the power tube. I find the o-ring usually half popped out of it's groove and around the skinny part of the rear bolt. The rear bolt is totally stock and unmodified as is the power tube. After the o-ring pops off too much gas leaks out the back of the power tube and velocity drops way off and the bolt doesn't reset for the next shot.

These guns are identicle except for a slight age difference.