By Lawrence Hansen
If you’re not an AR-15 fanatic yet, you might’ve wondered what Mike Searson meant when he referred to the barrel on his XM177 reproduction as a “Skinny Mini”. Maybe you’ve also heard AR aficionados talk about “pencil” or “A1” barrels. All of these terms refer to the external shape of the barrel on the original U.S. military-issue M16A1 rifle of the 1960s. Remember: Eugene Stoner designed the rifle to be lightweight and maneuverable, so the average infantryman could carry one for miles and still have the strength to fight. Consequently, the A1 barrel has just enough ordnance steel in it to do its job. Don’t worry - these barrels are plenty strong and will gladly let you send scores of thousands of rounds through them.
Even though the weight goal was achieved, the military found a few problems: under sustained fire - remember the M16A1 was a full-autocapable weapon - these barrels suffered heat warpage with a consequent bad effect on accuracy. Also, under extreme field conditions, they could actually bend if struck in just the right way.
When the Marine Corps adopted the M16A2 in the early 1980s, it sported a barrel that’s thicker ahead of the gas block/sight tower to resist heat warping and bending from impact. One tell-tale sign: an A1 barrel is .625" O.D. at the gas port, while an A2 is .750". A true A2 barrel is actually no thicker than an A1 aft of the sight tower, though many commercial barrels now have extra meat there, too, often called the “HBAR” after the Colt rifle it originally appeared on. The A2 is, of course, slightly heavier; if you’re loading up your AR with a lot of accessories, the few extra ounces probably don’t make that much difference.
Note: barrels are sold as either assemblies with the gas block/front sight tower installed, or “stripped”, minus the tower.
For some of us, a pencil-thin A1 barrel emerging from a triangular Vietnam War era handguard is the iconic image that shouts “M16”! Unless you’re shooting full-auto or dumping a 30-round mag as fast as you can in semi-auto mode, heat warpage probably won’t be a problem for you, so an A1 can serve you just fine.
The Gist of the Twist
“Twist rate” refers to the distance the rifling takes to make one revolution. Spin imparted by the rifling gyroscopically stabilizes the bullet for accuracy, just like putting a little spin on a football.
* 1:12" - best for 40 to 55 grain bullets.
* 1:9" - most common and best for standard 40-62 grain bullets including the military M193 round’s 55-grain FMJBT.
* 1:8" - ideal for 62 to 77 grain bullets, including long, steel-core bullets like military M855 round’s projectile.
* 1:7" - optimal for heavy, super-long-range 77 to 90 grain match bullets; handles military 55- and 62-grain projectiles well enough.
The reality is that for much of the shooting done with AR-type rifles, any twist rate will be serviceable. You will want to be more selective for specialized applications.