Well, I've been building up my AR15 collection to new heights, lately...

As of now, I have three uppers. Two from Spikes (a .22 and a 5.56mm), and the .410 upper that Turkish company is putting out. I have magazines for the .22 and 5.56mm, but I only have one for the .410 (they should be more imported by the end of August).

Now, I'm really not in the mood to get more magazines for another upper. I also figured I wouldn't want a caliber that works with the .22 or .410 magazines, so my platform is the 5.56mm one.

I have been looking at big-bores, and from what I read, either the .50 Beowulf or .458 SOCOM will work with 5.56mm magazines (I read that PMAGs using .458 will need to be modified, for whatever reason; if this pertains to metal magazines, as well, please let me know). I liked both, but have been leaning towards the Beowulf lately. Are there any other calibers that work with these magazines?

Some report being able to use .50BEO in unmodified 5.56 mags, while others have said they need to tweak the feedlips a bit. ~6 rounds of 7.62x39 will work from an unmodified 5.56 mag.

I've never had to modify magazines to get them to work with my .50 Beowulf.  Including PMags.

I have been shooting the 325gr Speer HP bullets so keep in mind that other bullets may have different results.

I have been using 20 and 30 round p mags with no modification for my 458 socom . 300 grain hollowpoints I also have been using 20 round 30 round and a 10 round 5.56 metal mags with no mods.

Requires 6.8 SPC mags