Piston Power
We Get Intimate With Faxon Firearms’ Switch-Barrel AR-15 Upper Prototype

What does the Mars Rover have in common with a piston-driven AR? Read on and find out…

Despite their recent surge in popularity, piston-driven ARs have been around for a long time. Colt was the first company to try solving the drawbacks of the original direct impingement (DI) operating system, way back in the decade that good taste forgot. Since the ’70s, numerous others have jumped on the bandwagon, with most of them riffing on Colt’s theme of a short-stroke piston acting on a modified AR bolt carrier. Due to design constraints imposed by the DI upper receiver, all of them swap the old problems of heat and carbon buildup for the new issue of carrier tilt, with some manufacturers addressing it more effectively than others.

Solving hard engineering problems is what Bob Faxon does for a living. Faxon’s company developed the parachute system for the recent Mars Rover, so you could say he has some experience in building gear that works when there’s no chance for a do-over. Bob’s also a shooter and it was only natural for him to found Faxon Firearms. When he looked at existing piston AR designs, he figured there had to be a better mousetrap. The biggest problem any AR piston gun has is caused by trying to cram a different operating system into the original upper receiver, so this was the first thing to be ditched. Once freed from its constraints, Faxon was able to design an upper that incorporates hardened, full-length rails for the carrier to ride on, completely eliminating the problem of carrier tilt. The new receiver design also had enough room inside it to accommodate a recoil spring, removing the need for the AR’s buffer tube and making a folding stock possible.

We tested the first preproduction prototype to be let out of the Faxon shop, and there’s a lot of innovation and hard work gone into this design, so let’s examine it from front to back.

The muzzle device is an A2 flash hider with standard threads, so a screw-on suppressor will fit without modification. Two barrel profiles are due to be offered: a fairly hefty medium contour and a lightweight pencil barrel, both in 16-, 18- and 20-inch lengths. The 5.56-caliber barrels will feature a 1:7 twist and the 300 BLK will utilize a 1:8, so that the widest range of bullet weights can be used and so the user can practice with lower-cost ammo before, say, heading out to slay hogs with the more expensive chambering. Both barrels are made in-house from 4145 pre-hardened chrome-moly steel and there are plans to nitride the production versions (along with the bolt and carrier) to increase their life span.

An adjustable gas block is permanently installed 9 inches from the chamber and has settings for normal and suppressed operation. Because this is a piston gun, the benefits of a free-floated barrel can’t be fully realized, so the Faxon team took the approach of keying the gas block into slots cut into the monolithic upper. The theory is that this approach stabilizes everything to the rear of the gas block and adds rigidity to both the barrel and handguards, but because the key slots allow for independent expansion as everything heats up, point-of-impact shift is minimized.

For the rest of this article, subscribe digitally here: RECOIL Issue 6

NEXT STEP: Download Your Free Target Pack from RECOIL

For years, RECOIL magazine has treated its readers to a full-size (sometimes full color!) shooting target tucked into each big issue. Now we've compiled over 50 of our most popular targets into this one digital PDF download. From handgun drills to AR-15 practice, these 50+ targets have you covered. Print off as many as you like (ammo not included).

Get your pack of 50 Print-at-Home targets when you subscribe to the RECOIL email newsletter. We'll send you weekly updates on guns, gear, industry news, and special offers from leading manufacturers - your guide to the firearms lifestyle.

You want this. Trust Us.