I purchased a Bushmaster Mod XM15-E2S lower that I planned to combine with an Alexander Arms .50 Beowulf upper that finally arrived today.  The only problem is that magazines don't lock into the magwell.  I tried the Alexander Arms mags in my S+W lower and they work fine, but when I tried the .223 and .50 mags in the Bushy lower, no luck.  I'm a relative newb when it comes to this so any help as far as how to fix the problem would be greatly appreciated.  Thanks in advance.  BTW, I'll post pics in a little bit.  The upper just arrived 10 min ago.

They go all the way in, with or without the upper.  The mag catch won't lock in the hole on the magazine to keep the mag from falling out.  I've tried tightening and loosening the mag catch but no luck.  Also, the mags are empty.

When I puch the mag catch in, it does spring back, so I don't think it's missing a spring.  And I didn't build the lower, I got it at a gunshow, which was also prob a mistake.  I might try to take the magcatch from the good lower and try it on the bad lower. Thanks for the advice.

I had a bushmaster lower that was too tight. certain magazines wouldn't fit in it and I had to "hammer" a 9mm magwell block in it.