by Hoot » Mon Sep 17, 2018 1:44 pm
I understand you don't want to use the 450b upper on your other lowers on a permanent basis but there's nothing to lose trying the upper on them for a few shots. If the problem persists, it would cement your concern that this is a gas system issue. If it doesn't persist, then you know better than to try and drill your way out of the problem. I get confused with my suggestions as there is a similar thread going on.
Did you look at the upper screw that holds your butt plate to your stock, to make sure it has a hole through it into the interior of your receiver extension? it allows pressure built up in it to escape when the bolt pushes the buffer back. I'm a big stainless steel socket cap head screw fan and had mistakenly replaced my vented screw with one. The result was almost identical to what you're experiencing right down to the BHO locking on the carrier instead of the bolt. Swapping lowers is an easily reversed troubleshooting tactic. Once a hole is drilled, you're stuck with it. I've drilled barrels to get them to cycle. Once I did that in error when the real problem was the chamber was tighter than the SAAMI specification. Wound up buying an adjustable gas block to undo what I had done.
Just sayin'...
Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.