You haven't said whether you were firing it on semi or auto. The barrels are all 'fitted' or 'matched' to the gun so you could have excess CHS. There are different regulators but they all operate in the same way and it SHOULD function the gun on the smallest gas hole but the second smallest for reliable functioning.
|You say in Q1 that you didn't have the gas regulator installed. Are you sure? It would fire but it simply wouldn't function at all if you didn't have one.
Fitting barrels PROPERLY (as opposed to just 'fitting a barrel') is quite complicated even for the average skilled Armourer as you need a selection oif barrels to start with, barrel locking nuts, locking shoulders, engineers blue, a few cups of coffee, the radio on to your favourite station and a new box of patience.