Are you crazy about AR15s? Maybe you might be in the market for a new upper to dispatch varmints in your backyard, farm, or to take out to your favorite spot for coyote hunting? The Franklin Armory F17-VS4 upper may be the right choice for you and all your varmint hunting needs! I spent a few months handling Franklin’s rimfire upper receiver, and tested some popular, easy to find ammo.

An Intelligent Design

The Franklin Armory F17-VS4, top to bottom, is a very well thought out design. The upper receiver is compatible with any mil-spec pattern AR15 lower receiver. The bolt carrier group (BCG) looks like other standard BCGs but the bolt face is drastically reduced in size to accommodate the miniscule .17 WSM rimfire cartridge. The upper was redesigned by Franklin from the original Eugene Stoner design and is no longer a direct gas impingement system. It is a gas piston operated system, and the gas key is replaced by a flat surface for the operation rod to engage. This piston is what cycles the action, instead of gas pressure that floods the internals of the bolt carrier group to unlock the lugs from the barrel extension.

-Manufacturer’s photo of the F17-VS4 upper with 20” Bull Barrel.

Being that most rimfire cartridges can dirty the action, this is an excellent choice of design in my humble opinion. Instead of a blowback design like other .22 LR designs, the added gas pressure with a larger case capacity in the .17 WSM allows enough gas pressure to syphon from the gas port into the piston system. The dirty, carbon filled gas is never cycled back into the bolt carrier group or into the action. Another favorable design that I appreciated is the materials used in the construction of the F17-VS4 upper receiver. The 20” bull barrel with a 1:9” twist is constructed of 4140 Chrome-Moly steel and is nitrided. Nitriding is a surface treatment which hardens the outside layer of the steel in an ultra-hard shell for the first few thousandths of an inch. It is highly wear-resistant, decreases surface friction, and provides a rich, dark black finish on the steel. I personally prefer 4150 Chrome-Moly Vanadium steel, but in this application of a rimfire cartridge the 4140 CM steel will perform well.

The handguard is a 12” straight tube, and I assume made of aircraft grade 6061 aluminum. There are MLOK attachment points toward the muzzle end of the handguard to attach popular accessories, and a threaded stud mount if you would like to attach a typical hunting sling. At the middle of the handguard, there is texturing that wraps around the tube for a comfortable grip. The charging handle is a typical mil-spec standard latch. There is no muzzle device installed on this upper as the muzzle is not threaded. It has an 11-degree, recessed crown and when testing accuracy, it performed very well. The upper comes with an included buffer designed specifically for use with their BCG. Three 20-round magazines were also supplied for this upper, and they have been designed to be used with mil-spec AR15 lower receivers.

The Set Up

I tested this upper on both a rifle length receiver extension and buffer, and with a carbine receiver extension with a collapsible stock. The rifle length buffer tube and stock was an ACME Machine AM-15 lower receiver with a LUTH-AR MBA-1 adjustable stock. The trigger was a Rise Armament RA-434 drop-in trigger that provides a very crisp, short pull at roughly 4.5 lbs. On Franklin Armory’s website there is a disclaimer that specifically states that most drop-in trigger units will not properly work with the F17 upper. This proved to be 100% accurate. The light trigger and hammer springs will not reliably ignite the primer of the .17 WSM rimfire cartridge. While drop in triggers tend to be the easy route for getting a smooth, crisp, light trigger pull from the AR15, this is not something that you will be able to do with this rimfire, complete upper receiver.

-Author’s set up of the F17 upper, shot off sandbag rests.

I also tested the upper receiver on a Bushmaster lower receiver that used a carbine receiver extension and buffer. The stock was an MFT Minimalist, and the trigger used was an ALG ACT trigger. This trigger is a phenomenal single-stage trigger for the price, and it gives a shooter a very reliable ignition of just about any primer that I have ran into. Upon test firing the Franklin upper with the ALG trigger, I had reliable ignition of every round with no issues. There is also another disclaimer on the Franklin Armory website that states that a rifle length buffer and spring must be used for reliable operation. I did not find this to be the case as the upper performed almost flawlessly with the nearly 200 rounds fired from a lower with a carbine length buffer tube and spring. Obviously, YMMV, but I had success with this configuration.

-Manufacturer’s photo of the ACME 4-16x44 TR-MOA optic.

Ammunition used was American Eagle 20-grain Ballistic Tips, and Winchester 25-grain Ballistic tips. Both brands of ammunition performed at nearly the same level, and both had roughly the same amount of failures to fire. Even with the reliable ALG ACT trigger, some rounds would not fire. This is common with rimfire ammo, and failures between both brands could be counted on one hand. The optic that was used was an ACME Machine 4-16x44mm TR-MOA scope that is first-focal plane. The optic was mounted onto an Aero Precision 30mm cantilever mount. Adjusting the zero on this optic has always proved to be very easy and straight forward. Elevation and windage adjustments have positive and audible clicks, and from previous testing are accurate with their adjustments.

Time to Punch Some Paper

After sending some rounds down the pipe of the Franklin Armory upper to foul the freshly cleaned barrel, I began to do some accuracy testing at 100 yards. Getting on paper was quick and painless at 25 yards. Recoil is measurable to a kitten fart and shooting all day without a muzzle device is pleasant and muzzle report is not obnoxious. It is worth nothing though that after a cleaning, do not be surprised if the first round keyholes through the target. The light, short .17 caliber bullet, when it encounters oil, can destabilize easily. This only happened once though, and after the bore fouled, and oil cooked off, accuracy tightened up immediately.

-Ammunition used was from both American Eagle, and Winchester.

The first load that I tested was the American Eagle 20-grain Tipped Varmint. This ammunition is affordable at only $14.99 a box for 50 rounds at my local Wal-Mart. I did a few different 5-shot groups and finally a 10-shot group at 100 yards to test how well the barrel handled the heat. The upper was able to put up consistent MOA 5-shot groups at 100 yards. The average of the ten 5-shot groups came in at .943 MOA, with the worst being 1.3 MOA, and the best coming in at .76 MOA. For a short-range varmint rifle, head shots would be easily achieved on prairie dogs, or woodchucks in the east. Shooting a relatively fast 10 shot group, without letting the barrel cool, opened the group up a bit, but at 1.95 MOA, I feel that this is more than adequate for a close-range varmint rifle.

-Author’s 10-shot group with American Eagle 20 grain loads.

The second load that was tested was Winchester 25 grain HE Polymer Tips. It runs for about the same price as the American Eagle. At 5 grains heavier, this load would be slightly better at bucking the wind, but you do sacrifice velocity. Range conditions were very mild on the days that this upper was test fired but pushing past 100 yards, this load may prove to be slightly better for hunting. The average of all ten 5-shot groups also proved to be tighter than the American Eagle, but obviously YMMV. The average was .832 MOA, with the largest being 1.2 MOA and the tightest coming in at an astonishing .54 MOA!! Consistently, this load also proved to be MOA or better with 5 shot groups at 100 yards. Without letting the barrel cool, and firing 10 shots, this ammunition was able to print a 1.62 MOA group. With both brands of ammunition being suited for hunting and having explosive expansion on target, the Winchester 25 grain Polymer Tips would probably be my choice for dispatching woodchucks in my Pennsylvania fields.

-Author’s 10-shot group with Winchester 25 grain loads.

Pros and Cons

First and foremost, Franklin Armory has produced a fantastic rimfire upper receiver that can be dropped on an AR15 mil-spec lower. They have been masters at producing different products that are unique within the firearm industry. The complete upper receiver proved to be very accurate for such a small rimfire cartridge. Feeding, and extracting were smooth and showed no issues at all. Light primer strikes were rare enough for me not to worry about for a hunting rifle, especially when the fault could have been with the ammunition. The gas piston operation is a smart and well-designed system that was clean running and very dependable while it was in my hands. Recoil is practically non-existent, so all day shooting sessions are a breeze.

With that being said, unique does not always equal a “must have” item. With an MSRP of $1199.99, the upper alone is very hard on the wallet. Don’t get me wrong, I believe that you get what you pay for, and you do get a lot with the F17-VS4 upper. But you will also have to ask yourself if the upper is personally worth the cost for your personal needs. The .17 WSM is an awesome little rimfire cartridge, but you also can not expect it to perform miracles at longer ranges past 150 yards. For its size, and ballistic coefficient, the tiny little round will get bullied by the wind. For smaller fields, this upper and cartridge will perform well on groundhogs. If you are performing long range eradication of prairie dogs though, this may not be the best choice of tool.


  • Chambered in 17 Winchester Super Magnum Rimfire
  • 4140 Chrome Moly Steel Barrel
  • Salt Bath Nitride Chamber, Bore, & Finish
  • 20" Bull Barrel
  • 11 Degree Target Crown with Recessed Muzzle Crown
  • 1:9 Rifle Twist
  • Forged A4 7075-T6 Aluminum Upper Receiver
  • Hard Coat Type III Anodized Black
  • Forward Assist
  • Franklin Armory™ Free Float Handguard (Fluted & Vented, M-Lok compatible, and an Integral Bipod & Tripod Adapter)
  • Custom Designed 17 WSM Bolt Carrier Group
  • Gas Piston System Design
  • Included Buffer and Franklin Armory .17 WSM Magazine (20 round capacity)
  • MSRP $1199.99


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