The difference between a carbine and a full auto BCG is weight. A full auto BCG slows down the cyclic rate for full auto. That's all. As civilians that normally don't have access to a real-deal M16/M4, we don't have to worry about slowing down the cyclic rate to support full auto. The benefit of adding mass to the BCG is a decrease in perceived recoil.
More mass or resistance to overcome, the slower the cyclic rate, the less perceived recoil. Given this, you can decrease the perceived recoil by adding mass or resistance. Buy a heavier buffer (more mass to overcome). Buy a stronger buffer spring.
I'm not saying not to go and buy a full auto BCG if that's what you want to do. I'm one of the idiots who swapped out almost everything on a 15-Sport just because I was curious.
The AR-Restor hydraulic buffer decreases perceived recoil. The cycling feels smoother overall.
A Fail Zero nickel boron treated semi-auto BCG. Slicker, harder, and more wear resistant than chrome. Run with minimal or no lube. Cleans up with a little solvent and a wipe.
The one mod that is really truly worthwhile is a trigger upgrade. A good trigger is amazing.
Go check out the pic thread. I think I listed every crazy thing I did to my 15-sport. The only OEM parts left on it are:the upper receiver, barrel, lower receiver, buffer tube, buffer spring, safety selector, magazine catch. Do any of these "upgrades", other than the trigger, really make my rifle more reliable? Who really knows. I'm not out there cranking thousands of rounds downrange on a monthly basis. I'm not inclined to torture test any of my firearms.
What the modifications definitely did was lighten the wallet, satisfy my own curiosity, and made one heck of an expensive "affordable" rifle.
