I have a 20 inch lightweight pre-ban upper (old style with triangular handguards, 3 prong flash hider) and a newer heavy 16 inch upper with a Vortex flash hider. Both are mated to the SAME Colt pre-ban lower. The 20 inch upper actually weighs about a pound less than the 16 inch. However, the 16 inch has substanitally more recoil. Not that it hurts or anything, but it takes a LOT longer to get back on target. It is not a sight radius issue as both have the same ACOG on them (compact 1.5x). Why is the 16 inch recoiling more? Is it the type of flash hider, or is there more gas exiting the 16 inch barrel? Anyone else notice this?

Interesting.  I would like to know the answer as well.  I could start guessing as I have some background in physics, but it would only be guessing and I am sure someone here knows the real answer...(Tat?  You there?)[:)]

Go for it; guesses are good. My ONLY possible explanation is that there are higher pressure gasses exiting the barrel.

what buttstock do you have on the 16"?
in the army the M4's recoil more because of the collapsable short stock. the recoil spring is shorter and the buffer is lighter/ shorter/ smaller.

Yes, I think you're right. There is more flip; not recoil. Will replacing the vortex with a standard A2 flash hider (solid on bottom) help with the flip? Is there a better way to help solve the problem? The buttstock for both the 16 and 20 is the same lower, which is an old A1 (no trap door) buttstock; i.e. not telescoping stock. Same buffer and spring as full length A2.

Higher gas port pressures mean the gas operation of the rifle is more violent in the carbine barrel.  This causes the carrier/buffer to strike the rear of the buffer tube faster/harder increasing the felt recoil.