ODIN Works DMR XL AR-15 6mm ARC Rifle Gas System. This 18" rifle-length barrel system is built from 416R stainless steel in a DMR profile. Caliber is 6mm ARC. Barrel length 18 in, twist 1-7.5, threads 5/8-24. Gas system includes a tunable gas block and a non-threaded gas block design. Gas block journal size-length: 0.750 in. Finish: Stainless Steel. Gun model AR-15. Additional features: Non-Threaded. Package contents: ODIN Works DMR XL AR-15 6mm ARC Rifle Gas System. Includes bolt carrier group. Compatible with AR-15 platforms.