In case you're wondering, as I was, the Magpul 60 round drum works just fine in the MSAR E4.  The straight neck of it is long enought to lock in place before the body comes in contact with the stock, while this wasn't the case with a Surefire 60 rounder.

It's a bit of a reach around (ha!) to wrap your arm past the drum and reach the grip if you're 5 9 like I am - longer gibbon arm types wouldn't have a problem.

It's a nice package with the drum.  The ballance is good, and the weight is well distributed.

Got to love a 27 inch long, 16" barrel rifle with a 60 round capacity.

Change "gibbon arms" to "Plastic Man arms", and yeah, you might be able to reach the grip!

Nice. I wonder if the drum works in a TAVOR.