Here is my AR10 20" with CAR, as you can see I added a remmy R3 reciol pad. Just sawed off about 1/2"on the bottom to make it flat, then JB welded it on. You'll have to scuff the pastic on the pad real well before it will stick. Its been about 700-1000 rounds so far with no problems. I smashed the butstuck on the ground for a simulated FTE, and it stayed on.
The R3 also fits inside the A2 butstock on the AR10 T models. Might fit the AR15, dont know. You'll have to enlarge one of the holes to make it fit, I used a 3/8 bit. Took about 10 minutes on my 300 RSUM, and I used the same srcews that came with the buttstock. -Note you will have a 1/8 gap between the plastic bottom on the R3 and the plastic of the A2 stock . If your worried about it you could fill it in, with some epoxy/JB. I left mine.
My 458 will get the same treatment as the A2