Ok we need more information. Pics would be good too. How long is the barrel etc. I did exactly what you are talking about. I needed to acquire a Armalite carbine 308 gas tube to have the proper length gas tube. A standard 3/4" gas block is what I used. I do not recall if my rail is DPMS hi or low I will check it out when I get home. This was w/the20" barrel upper they sell. I am pretty sure you want a DPMS high rail but I will check in the am.