. Only thing that is odd to me is when the upper is off the lower or tipped up from pulling the back pin the bolt carrier will slider backward extending out of the rear of the upper. The spring and buffer hold it from doing this when the upper is installed on the lower. No gas tube on the 9’s. It’s the buffer to heavy or is there something wrong or missing on the bolt carrier. (Slides freely no latch tension when not on the lower.
Colt mag adapter ( fit in nice)
9 mm upper and bcg from AR15 Parts.com.
I don’t understand why you bolt is pushing back out of the upper - from your description - I’m getting that it backs out of the upper a bit when the upper is opened and muzzle down?
It should stay forward, but there’s no tension from locking lugs, it will freely slide out - until it hits the ejection port door if closed, or the rear of the race for the alignment block on the top of the carrier/bolt.