One other question. Is there much difference between a mid length AR15 or M4 carbine length weapon besides weights? Is there really any reason to go with one over the other if both have a 16 inch barrel?
Within the same barrel profile there's little difference in weight between the midlength and carbine length gas system. Midlength is ideal for a 16" barrel, carbine length was designed for 14.5" and shorter barrels. Rifle length gas was designed for 20" barrels.
If you look at the three gas lengths matched with their optimal barrel length you have about 7" of barrel after the gas port. When you deviate too much from optimal you can have problems.
Carbine length is very common on the 16" barrel but not necessarily the best choice. Midlength lately has become very popular.