You know what didn't change when you switched barrels? The size of reamer used to cut the chamber. You're two barrels into this project and that oversized crap ammo you're trying to use is still oversized crap another barrel later.
What these guys know that you don't, is that you didn't get your original BCG "broken in" to mate and function well with your receiver; which was further compounded when you opted for an even newer tighter BCG that's even less well "broken in".
You're the expert though, so best of luck beating your new paperweight into a functional rifle, sounds like you're going to have a lot of fun figuring it all out the hard way.
Just sayin'.