I have a rc1 556 and a 762.
For your cloning you want the 556, just do that one for now.
The mounts are awesome, and repeatability of POI, toughness, and mounting system is awesome.
The 762 is a good choice too, but don't get stuck on surefire for 308/6.5. It's a good choice, but the modular cans that do it all simply is a better cost advantage. My buddy has a hybrid, it does 9mm/45, and all the rifle cals. And it's light, and I think a little more quiet. The ability to stick it on pistol caliber carbines and pistols is fucking great. The weight on a scorpion or a kriss feels really good.
My 762 is exclusively on a 300blk, and it's quiet as a mouse. I hear people say that it doesn't do 300 good but I disagree, it's great.
It's a really tough heavy can, I can club a baby seal with it with confidence. I'd buy both again with out a doubt, but I do like the do it all cans, they are the most bang for the buck.
Word of a advice, get multiple 22 cans! They are the funniest ones!!!
That's the difference in 566 and 762 on a MK18, the 762 just is too big.
The bottom is the 762 on a AAC 9 inch 300blk
Good luck, hope seeing the difference in size on the guns helps