Welcome from AZ ! First measure : A. Length of gas tube. B. Length between gas block and barrel extension. Your gas system may be set up for an Armalite rifle instead of a DPMS rifle there is about 3/8" to 7/16" difference between the two systems. Also look at the under side of the upper receiver with the charging handle installed ,for the position of the gas tube. It should be in the center of the circles in the upper (tool marking). That's at start.