Tapered profile, Triangular design. 20 mm inner bore, compatible with SRS sport barrel spacers (2 pieces included). M24x1 muzzle thread, compatible with M24x1 silencers, or with M14x1 CCW thread adapter (SBA-SIL-07). M24x1 muzzle cap included. Per size   300mm Designed for 330mm AEG inner barrel (or 327mm GBB inner barrel, specific Hop Up unit and cylinder head coming soon). 420mm Designed for 420mm AEG inner barrel (or 417mm GBB inner barrel, specific Hop Up unit and cylinder head coming soon). 510mm Designed for 510mm AEG inner barrel (or 507mm GBB inner barrel, specific Hop Up unit and cylinder head coming soon).