new AR15 build from used parts - occasional failure to grab a round

[adamge]

I have a couple AR15s.  One is my primary 3gun AR with about 4k rounds on it, super reliable, AP receiver set.  My other is a spare / trial gun, Wilson Combat receivers, I picked up and put a few rounds through it to determine that I preferred my primary.  But one thing I prefer on the spare is the receiver fit (almost no slop). 

 

Last night I stripped both guns bare and put all my primary parts into the upper and lower of the trial gun.  I took it to the range today for function testing, gas block tuning, and zeroing.  The only problem was that, about 3 times, each time on different brands/models of magazines, when I started with the gun unloaded bolt locked back, put a mag in, and pressed the bolt release, then went to fire, it goes click. I pull the mag and find no round in the chamber, and the top round in the mag has a full length scratch from what looks like the bolt lug.

 

I'll admit that these are crappy mags we are forced to use here in Canada.  One was steel and another was aluminum body.  But I've never had this happen before.  The magazine was fully seated in the gun every time, it wasn't a scenario where I failed to get a full engagement of the mag catch.  And I was loading with bolt back and not in a rush, so no chance I didn't get it fully on the mag catch.

 

Trying to figure this one out, I noticed that the mag has a fair bit of play (rocks left/right, and front/back on the mag catch pivot point) when it is seated in the gun with bolt locked back.  Should I try swapping mag catches in the lower to my other one?  Should I build up the magazine body with tape to try to take the play out?

 

If I can't get this solved I'm going to have to go back to my old primary build, because I can't have a 3gun AR that 20% of the time fails to grab a round when loaded from a locked back bolt.

[DesertTortoise]

Are you loading your magazines all the way up to max or leaving one or two out? They tend to work better downloaded by 2

 

Could be you need to adjust the tension on the mag catch

 

Maybe try your other lower on the Wilson upper and vice versa to isolate the problem parts