Is there a difference in muzzle velocity between 2 16 inch barrels where one has a carbine gas system but the other has a midlength system (and all else being equal)?

Just thinking about it, it seems possible, but I don't know if there is an actual difference.  Theoretically since the gas port on the middie is further out, the pressure should be higher in the barrel at the position between the gas port locations.  But I don't know if this is enough to change the velocity coming out of the barrel.  Anyone know?

Between a colt 16" carbine and rock river 16" midlenght my Chrono says no noticeable difference. The difference in the mean average was around 10fps in favor of the Colt. I get differences like that between identical barrels. Both were pumping out rounds at approx 2950 fps from the muzzle with M193. The boys over at Accuracy Speaks have said that completely removing the gas block will have no measurable affect on velocity so don't concern yourself about muzzle velocity but on function. The midlenght system makes the rifle feel as if it functions a little smoother although I can't say more reliable as I have never had a reliability issue with a Colt carbine except for magazine issues. I also like the the extra real estae a midlength RAS provives.

If there is a difference, you'd likely see a bigger difference simply from barrel to barrel. They are all slightly different inside. Some are a little smoother than others, some have cleaner rifling than others, Some will have tighter bores than others, the faster the twist is the more resistance to the bullet going through a barrel, etc.

No two barrels are gonna give the same velocity regardless of the variables, so get what you get and don't worry about that aspect of it.