Not sure. Seems like a couple things...I think hinge pin is slightly small, lower hinge pin holes are slightly large, and possibly the latch plunger needs to be replaced with an oversized unit. I was worried the upper receiver was out of spec but swapping para lowers resulted in a very tight lockup on one and a slightly loose on the original suspect lower...so it's something in therrWhat's wrong with the lower? @KERTWOOD is the resident FAL expert...