Q3131 (no longer produced, domestic ammo) and Q3131A (still produced, in Israel) are exactly the same, except for the model number.
On a side note, if the DPMS barrels are 5.56mm, why can't you use SA, or Q3131A, XM193, or what have you? All of it's 5.56mm, and should feed/fire/eject in a rifle with a 5.56mm chamber? That's one of the reasons that people buy AR15s. The ammo choice is pretty good, and there's lots of it (surplus too) available.