I just finished building an AR.  I have not fired it yet, but 1 issue I see is that the bolt catch won't hold the bolt back as well as it should.  It will hold it back, but if I slap the rifle, the bolt will slam forward.  It appears to just barely hold it.  Just sitting the rifle down will cause the bolt to go forward.  I have a fail zero BCG, and my lower parts kit is an AR stoner.  I used a houlding precision HPF-15 matched receiver set.   I took my BCG and put it in a different rifle, and it and it works in a different rifle.  I also too a BCG from a different rifle, and put it in my new build, and it still slams forward way to easy.  Any idea?  thanks

Just a little more info.  It does not work with an empty magazine in it, or without a magazine.  Without the empty mag in it I did press the bolt catch to where it hold the BCG back, but all I have to do is set the rifle down, or tap the butt of the rifle on the ground, and the BCG slams forward.  Hope this makes sense.

Sounds like the bolt stop is out of spec. Instead of swapping BCG's, swap complete uppers as well to see if there is a difference. Lock the bolts back on both rifles and compare the two.

what spring do you have in it?
a rifle spring in a carbine will have similar issues

not saying how i know

This.

I put a lower together with an old bolt catch from God knows where.  I thought I had a gas problem.  I thought I had a buffer problem.  Finally looked at the bolt catch and the "tooth" that is pushed up by the mag follower was visibly shorter than any other AR I had.  Swapped out.  Problem fixed.

OK, I figured it out.  I do have a carbine spring, I had to buy it after I bought a VLTOR buffer assy and spring.  (that was a waste of money, anyone need one for a rifle length buffer tube).  But my buffer assy is one that goes in a rifle length, not carbine length buffer tube.  So my BCG is just not going back far enough.  It is close, but needs to go back just a hair further.  I could probably shave a little off of my buffer assy, but I just put one on order, and will have it in a week.  thanks all

I figured it had something to do with the buffer tube, because I had the exact same issue. I built a rifle with an A1 stock and A2 stock screw, not knowing that there was a difference in stock screw length. Carrier travel was impeded, and the rifle would not lock back properly. Cutting the A2 screw down to length solved the problem. Fortunately, I solved it while function-checking after assembly, without a single live round being fired.