What are the differences between the gas systems of 20" rifle and 16"/14.5" carbines with standard FSBs?
Diameter and length of gas tube?
Size and configuration of barrel gas port hole?
Diameter is the same.
Length....there are four gas system lengths. Rifle, usable from 16" (custom only w/ .110" gas port) to 26+", Mid length, for 16 or 18 inch barrels. Carbine, for 10.5 to 16 inch barrels, even some 18s. And super short for 6.5 and 7.5 inch barrels.
Gas port diameter varies based on barrel and gas sytem length.