I just got a Beowulf upper but no magazines.  I have heard various sources claim that standard mags and even ploy mags work just fine.  Well I tried some basic testing with some polymer mags.  No live fire tests, that will wait until spring.  I loaded up a Magpul M2, a Troy Battlemag, and a Hexmag v2.  In each case the rounds when fully loaded were significantly pushing up agains the feed lips.  This started almost immediately with the Troy, at about half a mag with the Hex, and at around 7 rounds with the Magpul.  I then decided to smach the magazines around.  The Troy would spit rounds out with just a little wiggling.  A couple solid wacks with my palm on the side of the mag knocked the top round out of the Hexmag.  The Magpul took six or seven wacks on the side to spit out a couple of rounds.  None of these results fill me with much confidence.  I did not have an aluminum mag on hand to mess around with at this time.  Anyone else with a Beo have any advice?  Do regular Aluminum mags work find?  I would hate to buy expensive Alexander Arms mags if I dont have to.

I use lancer mags with my 458 socom and my 50 beowulf .

I tried GI mags, Modded GI mags, magpul , hex mag and Troy. None would feed on my 50 beo. I found some old thermold and orlite mags in my stash and oddly enough they feed all 10 rounds. People have posted about many different mags working but I guess you only know till you try them.

The problem I had with the GI mags were the bolt would strip the round out of the mag but the head of the cartridge would not slide up into the bolt face. It would slam the round into the chamber would the bolt itself was still out of battery.
I tried the upper on different lowers thinking maybe that was the cause. Same result on 3 different lowers.
The magpuls did the same.

Ok, I will have to pick up some lancers and some thermold.  And come spring see what works.  May have some that are range only mags, granted the beo is nothing but a range toy anyways.  I was disappinted with the hex mags because the company literature said they were gtg for beo.

GI mags and Lancers are what you want. At least with 458S magpul mags will not work because of the rib that goes down the front of the mag in the center. Any mag company that copied this design from magpul will also not work. Lancers don't have the center rib, and neither do GI mags.

If you can find them, the straight mags for the .499 Leitner-Wise work well too. I got several when some company was closing them out several years ago. Since .499LW has almost exactly the same dimensions as 50 Beowulf they work really well. Basically the same length as a 30-round GI/Magpul/etc. but without the curve.

Well, range report.  Tried the usgi and lancers.  Most rounds would not seat fully agains the bolt face and fully chamber.  When it did chamber I could fire one round and then would stovepipe.  Will have to grab a Alexander arms mag and give it a try.  Then maybe modify a usgi. Will post when I know more.

Honestly I have good luck with Orlite and Magpul magazines in my Beo. I do not like GI mags since they scratch the brass...