Stumbled across this thread from googling yesterday:

http://www.ar15.com/forums/t_3_118/551542_Is_this_receiver_gap_acceptable_.html

Can't reply since it was archived, so here goes:

The OP from the linked thread states that his lower was a PSA, but ultimately attributed his receiver gap to the upper. I'm having the same exact issue with one of my PSA lowers, and my receiver gap looks about the same as the one pictured. The pins can be pushed in place without a ton of effort, but it still looks really bad.

I currently have 2 BCM uppers- one is mated to a RRA lower, and one is mated to a Spikes lower. No issues with either setup, no gap of any kind. I can put both of those uppers onto this PSA lower, and the gap looks like it does in the picture from the other thread. So both of my BCM uppers are out of spec? I think not. Guess I'll have to order a PSA upper in order to not have a gap between the receivers (although that isn't what I originally wanted to do).

So if the takedown pins lock in place without a lot of trouble, the firearm is safe to shoot, and the buffer retainer pin shouldn't take a battering? How much gap is too much?

Bueller?

when you seat the upper to the rear takedown pin, if your bolt carrier slightly presses your buffer back, the gap size doesnt matter.

if your bolt carrier doesnt make contact with the buffer... that is when that detent will take a beating and you need to look at fitment.

+1 people put to much emphasis into the space or wobble of upper to lower fitment its rediculious

+1

If the gap bothers you, bed the lower to the upper.

Or just spend a few bucks and get an accu-wedge if it bothers you that much.

wedge will not make the gap smaller, only larger.

+1, I think the other guy was talking about the gap that causes receiver wobble. I could care less about that. One of my other AR's, I can almost squeeze a credit card into it, and it has shot like a dream. This setup locks in super tight, but the gap is just huge.