[Flatlander]

You should have no problems going from the m16 length DOWN to the m4 length (assuming the cylinder is the proper one for the longer barrel - probably is). I do this all the time with my m4/SPR with no serious side effects except *maybe* some fps loss - I just keep in the non-ported cylinder and swap inner/outter barrels around to whatever suits my fancy that day/week. You won't experience suck-back this way; possibly the other way around if you use the ported with the long inner barrel.

I tested the ported cylinder theory twice; here's what I found:

- G&P m4: LOST ~15 fps by switching the proper ported cylinder for a non-ported cylinder.

- A&K m249: Switched the improper non-ported cylinder for a properly ported one and saw NO fps change.