September 09, 2013
By Gus Norcross
Years ago, I bought an Armalite AR-10 rifle. Admittedly, I'm not a huge fan of the Armalite or DPMS .308 AR platforms. The early magazines didn't feed well, and the rifles seem big and awkward to me.
In any scenario where I really needed a .308 semi-auto rifle, I would feel more comfortable with an M14. But I bought one with a 24-inch heavy barrel and it shot reasonably well — although I wasn't happy with it in its original form. As the years passed, I gradually modified the big AR to better suit my needs.
Recently, I decided the as-issued A2 buttstock had to go. I wanted a collapsible stock so I could adjust length of pull and increase comfort. You can't simply install an AR-15 collapsible stock kit on an AR-10, due to the length and mass of the .308 bolt carrier. The specific parts needed are detailed below.
Receiver Extension: Also known as the buffer tube, the receiver extension contains the action spring and buffer. When the rifle is fired, the rear end of the bolt carrier travels into this tube a certain distance before it moves forward again into battery, and the length of the AR-10 bolt carrier assembly requires more travel than a standard 5.56 mm carrier.
Armalite employs a standard AR-15 receiver extension on its AR-10 rifles but gains extra travel with a shorter buffer. The AR-10 rifle buffer is the same weight as an AR-15 rifle buffer but is ¾ inches shorter.
To maintain this extra travel with a collapsible stock, we will employ a slightly longer carbine receiver extension. Vltor Weapon Systems markets a special tube for their A5 stock kit (part no. RE-10) that works on the AR-10 perfectly. It is about ¾ inches longer than a standard M4 carbine extension.
Buffer: The AR-10 carbine buffer is actually an extra heavy H3 AR-15 carbine buffer that employs three internal tungsten weights to achieve the same weight as the rifle buffer in a shorter package.
Action Spring: Armalite AR-10 carbines utilize the same action spring as the rifles, so in my case I already had the spring I needed.
For a buttstock I chose the excellent Vltor Enhanced Modstock, but any AR-15 carbine stock will work with this setup. Keep in mind with the longer buffer tube, the carbine stock will not collapse fully but it will adjust shorter than a fixed stock.
As my AR-10 evolves, I'm starting to like it more and more. In future articles, I'll cover trigger and handguard mods for the AR-10.
Comparing Receiver Extensions
Here are three AR receiver extensions: the rifle extension (top), the Vltor RE-10 extension and a standard M4 extension (bottom). The RE-10 is about ¾ inches longer than the M4 tube.
AR-15 rifle buffer, 5.2 ounces (top); AR-10 rifle buffer, 5.4 ounces; and the AR-10 carbine buffer, 5.4 ounces. The AR-10 carbine buffer employs three internal tungsten weights.
The increased length of the .308 bolt carrier assembly (top) compared to a standard AR-15 carrier requires a longer receiver extension and/or a shorter buffer.
Vltor Enhanced Modstock
The Vltor Enhanced Modstock will not completely collapse on the longer Vltor RE-10 receiver extension but most shooters will never operate the rifle with the stock in that position.