I bought a pre-ban 40 rnd mag (just for fun) to see if it would work.  If something is too far out of spec, would it ever wear something improperly, or cause unnessecary friction?  I've yet to try it(as I had to send my upper back to BM) but I remember when I tried to put the mag in, I had to open the bolt to get it to latch in correctly and then close the bolt (hinting it's putting pressure somewhere it shouldn't).  This I feel is a stupid question (especially given the mags are aluminum), but why is it this kind of junk seems to bite me in the ass if I don't ask for sure.

Gundraw

It won't hurt your rifle.

-- Chuck

I have to have the bolt open to put in the stock bushmaster magazine that came with the rifle if there are any rounds in it.  Don't worry :)

yeah, thats because you probably loaded all 40 rounds into the mag.  if all 40 rounds are in the mag the follower/spring cannot compress the extra millimeter or so to engage the latch.  hence you have to open the bolt to allow the mag to seat.  load only 39 rnds and try to put it in with the bolt closed...it should work.
some 30rd mags have the same problem.

Yes, i have heard of that, but I always short my mags 2 rnds.  So that mag only had 38 in it.  But I believe what you say, I've heard of that before.

Thanks, I'm sure it's fine, just Mr. Anal here having to worry about something.

Gundraw

Are you sure you put 40rds in it . i have one ,but you can put 43 rds in it ,but then it will not seat completey ( almost broke my toes when it fell out). so when i load it i count  to make sure i only put 40rds,then it works great.

Isn't it possible the spring is backwards and binding?

Have you taken that particular mag apart?