Quoted:
Rear takedown pin detent installs from the bottom of the receiver instead of the rear.
It is retained by the pistol grip instead of the stock.
I have just finished an AR10T build and this seemingly small difference in detail has me wringing my hands now (I have to work on not being so anal). Anyway, for whatever reason, Armalite decided to index the take down pin detent at 6 o'clock. Doing so obviously positions the detent "channel" in the takedown pin at 6 o' clock also. If you mic the takedown pin through the channel cross section you will notice the dimension is about .020" smaller than the perpendicular dimension. This causes the upper receiver to slop up/down significantly. Turning the pin 90° (detent and spring not installed) with the upper/lower connected removes the slop because the channel is no longer the "bearing surface" for the upper takedown hole.
My local gunsmith would not attempt to redrill the detent hole in the normal AR15 location due to the limited amount of material on the adjacent external surface. The possibility for bit walk and punching through the side of the receiver was too great (you will notice a raised area on AR15 lowers that adds reinforcement for the detent hole to prevent this).
My next plan is to mod (via dremel) my takedown pin with a slot in the head and a new selector style detent stop 90° from the current one next to the pin head. This will allow the pin to be pushed in as designed but then turned [ETA via 308 cartridge rim] 90° clockwise (viewed from starboard) to move the "channel" into the 9o'clock position and snug the upper down like it is supposed to be.
Maybe AR15barrels would venture into producing such a part to supply the AR10 growth industry.