I posed the same question. I have the 10.5" version and this was the response I received:
The carrier can be semi or auto. You should use a standard carbine buffer and spring for subs, you might want to try an H2 with supersonic.
Todd Krawczyk
Noveske Rifleworks LLC
P.O. Box 1401
Grant Pass OR 97528
541-479-6117
Fax 541-479-2555
www.noveskerifleworks.com
Two different friends are using 1) a JP low mass carrier and the other 2) is useing a modified spring, however, I think both of them have carbine length gas system.
Hope it helps.