Sounds like the length of the JP spring is just fine, then. If your rifle worked before the stock swap, it's either the buffer weight, spring rate or a combination of both. The other potential trouble area is the gas block being lined up. Usually, the opening into the gasblock is fairly generous so alignment doesn't need to be real precise. The last trouble area is ammo. It seems some M&P-10s don't like lower pressure ammo and using a higher spring rate/heavier buffer will only worsen that. I'm finding that's true with mine, but I need to do some more testing before drawing any conclusions.
With a good stock, I don't find the recoil of the M&P-10 objectionable. With the factory M4 style stock, it was uncomfortable for me because it dug into my flesh but the CTR with the thicker recoil pad took care of that. What I don't like about the CTR is the cheek weld I get when using a scope. It's half on the stock and half on the RE. This isn't a problem when using a taller mount & Aimpoint because I'm not pressing as much of my face on the rifle. The MagPul Moe Fixed will cover the whole RE. For me, that will be an improvement over the CTR. The choice is between saving weight, or getting a better cheek weld. Balance, even with the lighter M4 style stock is fine for me.
That doesn't mean I wouldn't install an UBR. It's just that I'm leaning more towards a lighter, less expensive option
With a good stock, I don't find the recoil of the M&P-10 objectionable. With the factory M4 style stock, it was uncomfortable for me because it dug into my flesh but the CTR with the thicker recoil pad took care of that. What I don't like about the CTR is the cheek weld I get when using a scope. It's half on the stock and half on the RE. This isn't a problem when using a taller mount & Aimpoint because I'm not pressing as much of my face on the rifle. The MagPul Moe Fixed will cover the whole RE. For me, that will be an improvement over the CTR. The choice is between saving weight, or getting a better cheek weld. Balance, even with the lighter M4 style stock is fine for me.
That doesn't mean I wouldn't install an UBR. It's just that I'm leaning more towards a lighter, less expensive option