Occasionally, when I use usgi 30 rnd mags, the last round will not be cycled into the chamber, causing the bolt to lock into the rearward position, leaving the rnd "floating" around in the upper.  Any help is greatly appreciated.  TIA--VT4ME

Shoot only with 20-round mags !

(Which is a smart a** way of saying I don't have a clue.)

As a pure guess, I would think this problem occurs only with 'some' of your 30-round GI mags.  Many, many mags are out of spec.  You might consider marking the mags you have problems with.  On a purely personal basis, I only shoot the old Sterling 30-round mags and those only in a F/A weapon.  When shooting my semi AR's I only use 20's - far fewer problems.

VT4....

This is a strange one, initially it sounded as if there is a problem on the bolt catch...BUT if it only happens with your 30 rounders I would start there, new springs, good followers(green)and a cleaning. Check feed lips to make sure they are not to wide allowing last round to flip out when no other rounds next to it. (unlikely but something to look at)
Post your findings.

Robert

www.rbprecision.com

I'm no guru, and I'm not really up to date with the latest in fixes, but if I shared this problem, then I'd ask: how many 30-round mags do you have, and does each demonstrate the same problem in a buddy's AR?  Also, if you try another "reliable" 30-round magazine, do you have the same problem?
Condition of mags, then followers and springs. . .  
That's where I'd start.
(Listen to the experts)
IMHO

Thanks to all who posted replies, I guess I'm gonna buy 5 niw 30 rndrs and 5 niw 20 rndrs to see if I can get to the bottom of this frustrating problem.  Thanks and take care--VT4ME