they drop free fine except when the bolt is locked back by the bolt catch.
It sounds like this is true regardless of the magazine. I want to understand what additional forces are at work that prevent falling when the bolt catch is engaged but aren't at work when the charging handle is locked. To try to find out, I removed my lower, inserted an empty mag, zip-tied my bolt to the top of the lower, then slid it forward until it engaged the bolt catch. I conducted several tests:
1. With no forward pressure on the bolt, the magazine repeatedly fell free.
2. Applying a small amount of forward force on the bolt, the magazine repeatedly fell free.
3. Applying a realistic amount of forward force, by manually compressing the recoil spring, the magazine did
NOT fall free during any of my attempts.
With all of the pieces and parts to manipulate, it was difficult to inspect the potential mating surfaces that could be causing the extra friction. My guess, at this point, is that the forward force applied by the retracted bolt is actually flexing the polymer lower just enough to add that little bit of friction needed to keep the magazine from falling free.
If I can work out some sort of jig that will hold the lower and bolt together hands-free, in the proper position, with a realistically compressed recoil spring, then maybe I can start poking around and see if the bolt is touching the magazine in any way.
Another thought I had was that maybe the bolt release lever is the culprit. Perhaps, when the lever is actuated, it's causing some friction that isn't there when the lever is at rest. I just don't know at this point.
