My .50 Beowulf upper just arrived about 10 minutes ago.  I'm slapped it on a Bushmaster Mod XM15-E2S lower and it fits great.  The only problem I'm having is getting the mags to lock into the magwell.  I have a 7 and a 10 round magazine that I got from Alexander Arms and I'm having the same problem with both.  I've hit the bottom of the mags pretty hard and pushed to get them to lock in, but still no luck.  Any suggestions would be greatly appreciated.  I will post some pics of the new beast later this afternoon.  Thanks in advance for the help.

ETA: I forgot to test the mags on my other .223 lower (a S+W MP15A), and it turns out they fit there.  Also, the .223 mags don't lock into the Bushy lower so that's obviously the problem.  I'll post in the main forum about fixes but again, any help on fixing the lower would be greatly appreciated.  

I don't know if you have ever built up a lower, but try tightening up the mag catch.

Push in the mag release with a bullet tip, and rotate the catch a few turns clockwise.

The screw in the center of the mag release should be flush up with the surface, not deep to it.

Thanks for the quick reply.  I've never built a lower, but I'll try that now and let you know.

No luck.  I'm wondering if it may be a problem with the mag catch because the mag goes all the way in, but just doesn't lock.  A quick visual comparison with the S+W lower, and they both appear the same, but I'm no expert.  Should I look to replace the mag catch?

It's possible the catch is bad, it is also possible that the mags are not seating all the way.

When you seat the mags with the upper off the lower, what happens then?

You can also mark the side of a mag with masking tape in the S+W and then put it in the BM to see the difference.

You can remove the catch and compare with another.

Yeh, try it with the upper detached just for the hell of it.  I know that sometimes if you max out the mag with the Beowulf, it does not like to seat all the way, so I leave one round out, but it sounds like you are just trying to seat an EMPTY mag, which is a whole 'nother aminal...

I am trying to seat an empty mag and no luck.  I tried it with the upper off and it still won't seat.  I'm going to try the tape thing to see if there is a difference, but when I compare both lowers without the uppers, it appears the mags go in all the way.  They just don't catch on the Bushy.  I guess that's what I get for buying a bushy.

I tried the tape trick and it the mag is going in the same distance on both lowers, the mag catch is just not locking into the hole on the magazing on the Bushmaster lower.  What now?  Thanks again for all your suggestions.  I really appreciate it.

I have been using all my old issued mags 30's and they (well worn) function fine in my beo

IShallNotFail, it is a mag or mag catch issue, not a 50Beo issue.  He tried all mags in the lower and they won't stay in, but do stay in on his other lower.  He also tried it with upper off the lower and it still won't work.