Would using a pigtail gas tube or a wider aftermarket tube on a 14.5" M4 reduce muzzle rize?  Was just curious because I have these Iraq war vids of Marines using the 20" M16s in semi mode, and they have essentially 0 muzzle rise.  In fact, looks like there's barely any recoil at all.  And this is without all the gadgets hanging off the handguards.  My 16" bull barrel has noticeably more muzzle rise when shooting freehand (XM193), and many other vids of 14.5"/16" ARs I've seen to be the same.  

Is it just my imagination or does the barrel length and/or gas tube length affect perceived recoil?        

Well they know how to hold the rifle to keep muzzle rise down for one and the rifle has more weight out in front to hold the muzzle down if its a 20" barrel.

The reduction in gas impulse from the different gas tube is there but minimal as the barrel, bolt, carrier, butt stock design of the M16 withh all those parts in line means this is all backward push and very little muzzle rise.  There is also a heavier buffer on the rifle and greater total weight of weapon.  Gas tube length is the smallest part of the equation.

It is not so much that the gas tube affects muzzle rise.  It does affect the recovery of the muzzle to its original position.  It does this slowing down/speeding up the cycle speed of the bolt.  I Have a 16" dissipator that I use for IPSC three gun,  with the comp on it it doesn't hardly move at all but it is harsh, and cycles very sharply.  I recently installed a pigtail on it and the cyclic rate slowed way down which, makes the muzzle recover even better than before, espically from unstable shooting positions or with hot loads.  In uncomped pistols the timing of the gun is everything,  this transfers to rifles also.

TTT for more opinions.  

What Huck said.

The general rule is that the longer the gas system, the softer the recoil pulse.  YMMV.