They're pretty good, but not standard-issue- I was just curious on the matter of the boxes. Actually, since M249s can't take 5.56 STANAG magazines (only taking just the boxes) anymore (I think Beta-C goes into a regular receiver), whereas the M249 uses a chain-fed lug), it might be worthwhile using a box is what I'm suggesting- about four or five would be sufficient for an Automatic Rifleman, because to be honest, they're there to lay down a shit-ton of fire, and I mean, the M249 can match 10 riflemen in suppression and firepower over a period of time. Up to you though- there older M249s that I think can accept STANAG magazines, but I haven't seen those for a while. I mean, I don't want to seem tedious and you know, anal about things like this, it's just imaging how the Beta-C fits into the M249. Tell you what, it definitely fits with an M27 IAR for example- and I could imagine it mounted on there, that's absolutely for sure.