Does anyone know if there are any modifications neccessary to the gas system when changing to the 18 inch barrel? It seems to me that there would be a definate difference in the duration of the gas pulse available to operate the action, given the 4 inch difference in barrel length. It may be that the volume of gas and the pressure are still well within the operating envelope. I ask because I'm gathering some parts for a M1a bush rifle build up. Years and years ago, I built an SKS carbine from a kit, and I had definate shortage of gas to operate the action, after cutting off some barrel. Thanks guys in advance.