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Barrels with chrome-lining and button rifling have chambers for both .223 and 5.56 NATO ammunition. They have a 1-8" twist to improve accuracy. The chrome-lining process keeps the bore consistent and hand-lapping ensures consistent lands and grooves. The gas ports are optimized for length and chambering for reliable cycling in carbine, mid-length, and rifle gas systems. The barrels have 1/2"-28 tpi muzzle threads for standard muzzle accessories and M4 feed ramps to aid feeding. Newer versions have a dimple that prevents screw-on gas blocks from shifting during firing. Hybrid barrels have a reduced and tapered contour under the handguard to reduce weight, while HBAR barrels have a .870" diameter for added stability and heat absorption.