EABCO 6mm ARC Barrel fits AR15 Rifles- This extra long EABCO 24" HBAR Accuracy Barrel® was designed to squeeze the most from 6mm ARC in velocity, performance, and accuracy capabilities. The 1:7.5" rifling twist is faster than most 243/6mm barrels and meets the original design specs for long range HEAVY 6mm ARC bullet performance. The 24" barrel gives the fastest velocities. And the Heavy Barrel (HBAR) contour give the most mass for a steady hold and recoil abssorption. See the Hornady website for all of the specs and details of the 6mm ARC cartridge itself. - Conversion of Your AR15 to 6mm ARC - It's very straight forward... Swap in this barrel. Swap in a 6.5 Grendel Type II compatible bolt. And, swap in our 6mm ARC magazines.