I can only manage to get 41 in an M2A1 can. Someone said I should get 57 (???) in there, and I see nowhere near that much space left. Are the XM193 boxes bigger than other boxes?

Yes, I find the XM193 boxes to be more difficult to get into a can, it's the little plastic devider.

I can easily get an entire case of something like Q3131A into a 50cal can.   With the XM193 I can only do like 800 rounds into a 50cal. can.

I had one lonely box of Q3131A sitting around so I checked it out; it is a good deal smaller than the XM193 box. I also have a box of Q3131 which is even bigger than the XM193 box. Oh well, I guess 41 boxes is the best I'm gonna do.

I just realized one of the ammo cans I'm using says "820 CRTG. 5.56MM / BALL M193 / CARTONS", well, I guess I'm not the only one who can only get 41 boxes in there!

Why dont you remove them from the boxes and load em on stripper clips?  Seems like you could get a lot more in there without the packaging.

You can get 50 boxes (1000 rds) of Q3131A in a standard .50 cal ammo can.