+1 to all of that...the spikes heavy buffer goes in all my rifles with carbine stocks. it does smooth it out a bit.
That being said, the recoil impluse is diffrent on a midlength vs carbine. not quite the transition of rifle to carbine, but you get the idea. If you wanted to smooth it out a little more corndoggy, you could always use a heavier bolt carrier group. I usually use a young mfg national match m-16 carrier in all of my ar's. The extra mass of the carrier slows the cycle a little bit, and gives extra inertia to slam that bolt home.