Besides the grip whats the diff between the A1 and A2 lowers?

There are some reinforcements around the rear where the buffer tube fits in and there the fence around the mag release, but I don't remember if these were added with M16A1 or M16A2.

The critical difference is that the A1 is stamped AUTO while the A2 is stamped BURST.

The military does reuse the old A1 lowers; they are restamped and then used to build A2 rifles.

I've got a pre-ban SGW lower, and as far as I can tell it's got all the A1 features.  This lower is from before OLY started making the double-letter-prefixed serial numbers, which as far as I can tell is when they switched to the A2-style lower.

Anyway, it seems like the A2 reinforcements were mostly to beef up the rifle's strength for grunt use.  Soldiers would supposedly use their rifles as prybars, stepstools, etc.  Is this pretty much why the reinforcements were added?  I can't really see how "normal" shooting would require the reinforcements.  In fact, aren't the current SPRs built from A1 lowers?  It seems like the A1 lowers are pretty strong - they served through Vietnam.

Oh yeah, one other difference between the A1 and A2 - besides the selector side, doesn't the A2 say "SAFE/SEMI/BURST" on the right side of the receiver (when looking down the barrel), while the A1 does not?

There are 3 major M16 lower receivers:  XM16/XM16E1, M16/M16A1, and M16A2/A3/A4/M4/M4A1.  Each is stronger than the earlier version.  Only major visible enhancement was the addition of the raised protective fence around the magazine release which is part of the M16A1 spec.  Sharp eyes can pick out smaller differences in radius and thickness.

There are no functional differences in these receivers and they're interchangeable.  The markings by the selector are cosmetic changes as the internal parts determine how the selector functions.  You can convert a M16A2 to M16A3 by replacing the BURST parts with AUTO.  Yep, it'll still say BURST, but will fire AUTO.  

-- Chuck