DOug, I think you mean rifle upper with an M4 barrel extension ( like Bushmaster, RRA and many did for years). If you do rifle extension to M4 upper I can virtually guarantee constant stoppages due to the incompatibility.
DOug, I think you mean rifle upper with an M4 barrel extension ( like Bushmaster, RRA and many did for years). If you do rifle extension to M4 upper I can virtually guarantee constant stoppages due to the incompatibility.