Barrels

A common rule of thumb: pistol gas under 10 inches, carbine for 10 to 16 inches, mid-length for 14.5 to 18 inches, and rifle length for 20 inches and up. A mid-length gas system on a 16-inch barrel is a popular, soft-shooting choice.

For 5.56, a 1:7 or 1:8 twist stabilizes the common 55 to 77 grain bullets; 1:9 works for lighter bullets. 300 Blackout barrels usually use 1:7 or 1:8 for both supersonic and heavy subsonic loads.

.223 Wylde is a hybrid chamber that safely fires both 5.56 NATO and .223 Remington, with the accuracy of a tighter chamber. A 5.56 NATO chamber fires both too; a .223 Remington-only chamber shouldn't be used with 5.56 ammo.

On a rifle, a barrel under 16 inches makes it a short-barreled rifle, which needs an approved ATF form first. A 14.5-inch barrel with a muzzle device permanently attached to reach 16 inches counts as 16.

Chrome-lined and nitrided barrels both resist wear and corrosion; chrome lining is the military standard and nitride is often a little more accurate. Bull (heavy) barrels stay stiff and accurate as they heat up but add weight, and carbon fiber barrels wrap a thin steel liner in carbon for a stiff, light barrel at a higher price.