I have a RRA lower and a DMPS upper. When combined, there is a slight bit of play between the two. They are not tight. Doesn't bother me really, since the play is very very slight, but still I wonder if this is because of the different manuf between upper and lower?

an accuwedge would solve all of the play between your upper and lower receivers. Just remember to squeeze the upper and lower together when pushing the rear push pin in or out. Just my 223 cents.

Armalite says in a Tech bulletin:

February 7, 2003
TECHNICAL NOTE 55:  RECEIVER TIGHTNESS (excerpt)-
"Receivers will normally exhibit a certain amount of looseness.  This is normal and will not affect accuracy.  Shooters may reduce the movement by insertion of a small amount of material between the receivers to improve the feel of the rifle, but it won’t improve accuracy."
© 2003 ArmaLite Inc®.

When I was in Basic Training, I had a very old M16A1.  It was VERY loose and rattled like hell.  I also needed to crank the rear sight almost all the way to the left to zero.  This beat up old war horse of a rifle shot great!  Because the sights and the barrel are all in the upper, unless the looseness effects the smoothness of your trigger squeeze, I wouldn't worry about it at all.

We were told this was perfectly normal, and I saw no reason to doubt.

Just my $.02,
Mr. L.

If it's loose enough to actually "rattle" I'd use an Accuwedge, or I would look for another upper or lower. If it's only a slight amount, don't worry about it as that's normal and doesn't affect the shooting in any way. Use of an Accuwedge that makes the fit really tight can, over time, actually increase wear on your lower takedown pin holes.

I remember reading that the Accuwedge is acctually bad for your rifle (it increases wear). The upper and lower is supposed to have a little play. I can't find it anymore though, but I'm sure someone else can chime in on this.