Centerfire cartridges are great, but sometimes a little rimfire lovin’ is what you really need to get the job done. Winchester has developed a pretty kick-ass rimfire cartridge in the .17 WSM or “Winchester Super Magnum.” It’s a higher-powered .17 caliber rimfire round with an insanely flat trajectory and a muzzle velocity equal to most 5.56 rounds. The caliber means you can pack more ammo and more power into the same space, but the only rifles set up to fire that new cartridge have been bolt action guns. Until now . . .

Franklin Armory are the same people who custom-machined the lower receiver I’m using in my 300 BLK SBR rifle, and I’ve been a fan of their design ever since. They have a penchant for outside -the-box thinking and nifty designs, and the F17-L is another example.

It all started with the caliber. Like I said, the .17 WSM is a great little round for varmint hunting, but when you set foot in California there’s another benefit that it has over the standard 5.56 round. In the Golden State, rimfire guns aren’t as highly restricted as centerfire guns. So while a varmint rifle in 5.56 would need to confirm to all of the ludicrously restrictive “assault weapons ban” requirements like bullet buttons and fixed stocks, an AR-15 chambered in .17 WSM gets to thumb its nose at all of that hogwash. The benefit to California residents (where Franklin Armory is located) is obvious, and so they set to work making it…well…work.

There are two huge hurdles to a rimfire cartridge in a semi-auto rifle: cleanliness and capacity.

Rimfire is the dirtiest of the ignition systems. Typically a .22LR rifle will be completely coated in carbon and nearly useless by the end of an extended range session, but with the F17-L, Franklin Armory has baked in a little extra ingenuity from Osprey Defense. Instead of using a straight blowback design like most rimfire rifles, the F17-L uses a gas piston system like many 5.56 AR-15 rifles. This allows the bolt to remain closed longer, which means lower pressures when it finally does open and less gas (and filth) blowing back into the chamber.

Out on the range, that piston system really showed its worth. After firing hundreds of rounds, the chamber (shown above pre-cleaning) was still pristine. If I had tried that with my rimfire AR-15 barrel, the guts of the rifle would have been pitch black.

The other problem to contend with using rimfire is in the magazines. Rimmed cartridges have a huge issue when it comes to feeding systems, as anyone who owns a Mosin Nagant will tell you.Rims not only make the gun malfunction if loaded improperly, but also require a much more pronounced curve to the magazine to load more than a couple rounds. The magazine well on the AR-15 platform was designed for the relatively straight-walled 5.56 NATO cartridge, so getting a mag with the proper curve to fit is a difficult proposition.

To solve that, Franklin put pencil to paper and designed their own proprietary magazine from the ground up. The version I tested sported an aluminum magazine design that holds 10 rounds in a semi double-stacked configuration, and seemed to work pretty well. However, I did have a few hiccups. There were a couple times when the last round failed to feed, and the process of inserting and removing the anodized magazine was a little difficult due to the friction between the parts. Franklin Armory has told me that this is a common problem and they’re working on some polymer magazines to address it.

On the range, the gun works as advertised. The AR-15 design has been around long enough to be largely perfected by now, so that’s no surprise. However, the little things about this rifle are what make it really nice. For example, the matching milled upper and lower receivers that are available in multiple colors are a great addition and sure to be a welcome option for hunters. The styling is pretty slick as well, with the flowing shell deflector and forward assist molded as one continuous piece. I appreciate little details like that. The trigger as well is much nicer than the standard “mil-spec” switch most rifle makers drop in their guns, and makes shooting accurate groups pretty easy.

The barrel is a thick piece of 4140 chrome moly with a nice target crown on the front for accuracy. This is California after all, land of the incomprehensible and ever shifting gun laws, so a muzzle device might not be the best first choice. With the popularity of silencers for varmint hunters elsewhere in the US, I’d love to see a model with a threaded barrel, but a standard target crown makes sense, too. It’s all about what you plan to do with the gun, and this one seems pretty well designed for killing large numbers of four-legged pests.

Surrounding that barrel is a solid free-floated tube, which not only isolates the barrel from hands and bipods, and also keeps the heat from the barrel from screwing with the sight picture. There’s another nifty party trick, too: a camera tripod adapter built into the handguard and perfectly positioned to balance the gun, so you can take well-stabilized shots even while standing up.

The effect of those design changes are pretty obvious on the range.

Shot from 100 yards, this group measures about 1/2 inch from center to center. The gun does this all day long with some good glass and a steady bipod, which is perfect for varmint hunters shooting small targets at great distances.

For a varmint rifle, the F17-L is pretty darned good. Firing slippery little rounds, the gun is both accurate and deadly. There are still some teething problems to contend with, specifically the magazine (Kevin Brittingham at SIG SAUER jokes that the best way to make a gun is to find a magazine that works and build a gun around it). The magazine in this case wasn’t quite up to the same standards as the rest of the rifle, but there’s a magazine re-design in the works so that should change shortly. But until then, the current product shows a ton of promise.

Franklin Armory F17-L Specifications

Caliber: .17 WSM (Winchester Super Magnum Rimfire)

Action: Semi-automatic Osprey Defense piston

Barrel: 20″ 4140 chrome moly bull barrel with 11 degree target crown

Charging Handle: Standard

Trigger: Franklin Armory custom factory-tuned trigger

Stock: Magpul MOE rifle stock

Handguard/Upper: 12″ TML M-Lok Fluted and Vented, Integral Bipod and Tripod Adapter

Grip: Ergo Ambidextrous Sure Grip Pistol Grip

Bolt Carrier: .17 WSN Salt Bath Nitride

Magazine: 10-round magazine made of custom Franklin Armory aluminum

Gas System: F17 Piston

Gas Block: Piston

Receivers: Olive drab green hard-coat anodized Libertas Billet lower receiver and Libertas Billet upper receiver.

MSRP: $1,999

Ratings (out of five stars):

Accuracy: * * * * *

1/2 MoA with a semi-auto rimfire rifle. What’s wrong with that?

Ergonomics: * * * * *

It works. It’s an AR-15 at heart, and the AR-15 is pretty great ergonomically.

Ergonomics Firing: * * * * *

Zero recoil, low noise, and a positive reset. The bolt even locks back after the last round.

Reliability: * * * 1/2

There were a few feeding issues, but a new magazine should fix that.

Customization: * * * *

It’s an AR-15. The only thing costing the gun a star is the proprietary magazine.

Overall Rating: * * * *

The gun still needs some work. But not only is it a nice varmint hunting rifle, it’s a gigantic middle finger to the state of California. What’s not to love about that? A little on the expensive side, though.

More from The Truth About Guns:

RECALL: Do Not Use These Lots Of Winchester Super-X .17 HMR Ammunition

Ruger’s New .17 HMR, .22 WMR Magnum Models of the Ruger Precision Rimfire Rifle