DO NOT GET A STAG! Everyone I've seen has been horrible, in every respect.
I do notice a difference between a M16A4 20" and the M16M4 16", but it's pretty minimal, I think alot has to do with the cheak weld and sight radius, with the RCO's minimal 4x32 power, most of that is difference is made up for. If I remember right muzzle velocity on the 20" is 3110 f/s and on the 16" is 2970 f/s. For the record, the M4 barrel is a bit less than 16", I think it might be 16" from the back of the chamber to the flass suppressor, the RRA is about 1.5" longer when the uppers are laid next to each other.
My Personal Rifle and Wifes
I have a RRA Entry Tactical as I just stated in another thread, 16" barrel, 6pos stock. With M885 SS109, other wise known as 'green tip', I've scored shots @ 750m with an ACOG, 10 back to back shots on a 12" steel target, she had 10 little holes blasted through her.
My wifes RRA NM, will hold SUB MOA @1000m, 5 shots on a 19" silhouette target with a 12" center circle, shots measured 7-3/4" at the farthest, from sand bags on iron sights, 77gr Blackhills, molly coated hollow points, mid morning dessert shooting.
I think 90% of rifles will out shoot the guy squeezing the trigger. RRA makes a great rifle. I think they're just as good as Colt.
Remember the US Marines shoot rack grade M16's at 500 yrds on iron sights scoreing on a 19" wide target, only useing a web sling for support from the prone. You're talking about useing a scope and have the ability to use support.