Have a new build that is funny about catching the bolt once a magazine empties. I am using Magpul Gen M2 magazines and in this gun and another Long AR15 they catch occasionally, but with a plain jane mil spec mag it catches overtime.  My question is there a more reliable magazine out there?  Also is there an enhanced aftermarket bolt catch that will cure the Magpul Mag issue? This bolt catch is one from KAK in the standard LPK.

Does it feel scratchy when you push it? Try removing it and massaging the bolt catch with some sandpaper.

I had a similar problem on one I put together the other day. Maybe it'll help you.

Your problem isn't the magazines. The magazines work great in to-spec guns.

Your gun... either a weak bolt catch spring (PSA, Radical, and other economy grade sources often have weak springs).

Something with the bolt catch is binding. Wrong spring, too small of a hole drilled to pivot on pin, lower scraping it, etc.

Or your gun is cycling either too slow, or (much more liekly for a 7.5" gun) too fast.

To speed up cycling, look at gas leaking from a loose gas key, too heavy of a buffer, misa;igned gas block, or adjustable gas block choking the system.

For too fast, best way is to throttle it at the gas port. Since that's already drilled, adjustable gas block is the next best way. Otherwise add weight to the buffer.

I had that problem with a couple of my ARs. In MY case, lancers and gen 3 mags solved the problem. Sold 'll my gen 2s.

Your mileage may vary, but the mag was the problem for me.