During their 8 years as a Federally licensed firearms manufacturer, Primary Weapons Systems (PWS) has continued to re-think and re-engineer the AR-15 rifle platform. Their long-stroke piston-driven design has managed to combine the best of Mikhail Kalashnikov’s AK-15 and Eugene Stoner’s AR-10. Although PWS uses the slogan, “Strength Through Evolution,” it’s probably more accurate to describe their philosophy as selective breeding, taking the best attributes of different types of semi-automatic rifles and producing an improved hybrid.
The most recent offering from PWS, which we review here, is their MK220. Chambered in .308, as are all the other models in their 200 series (the MK100 series are .223 caliber), it has a full 20″ barrel with a removable precision rifle compensator (PRC), an alternative to the carbines in the PWS MK200 series. Both the upper and lower receivers are CNC-machined from mil-spec 7075 aluminum forgings and hardcoat anodized at Type III mil-spec for corrosion and wear resistance. The rifle weighs 9 lbs., 7 oz. and has an over-all length of 39.5″.
At the heart of the rifle, of course, is the piston-driven system. The origin of this design dates back to the AK-47 and uses a separate piston, driven by gases bled off from a fired round, to push against the bolt carrier (which drives the extraction and ejection of the fired case). This is, of course, a different method than the gas direct-impingement (DI) system, which carries the gas through a tube directly to the bolt carrier key. The advantages of the piston-drive system, advocates say, are decreased chamber fouling and heating (since the piston is the only part coming into contact with the gunpowder-produced gasses).
The debate between the advantages of the DI versus piston-driven system has been carried on for quite some time – and will continue to be, as long as shooters remain interested in the mechanics of their firearms – but pistons are obviously here to stay. The U.S. military is even testing several piston-driven rifles as replacements for the M-4. PWS, which was originally named AK Concepts, is obviously on the side of the piston-driven system, but they’re not content to just accept the original design; instead, they’ve been working to improve it. By using a one-piece piston in a long-stroke design, it provides a pushing action, rather than a punch and reduces felt recoil.
The adjustable gas block also allows greater control over the amount of force provided to the piston. This not only makes for a quieter environment for the shooter (excess gasses exit from the muzzle, rather than being vented to the side), but increase the reliability of extraction and feeding. The gas block has four settings: normal operation (for standard civilian ammunition and some less powerful military ammunition), hotter loads (particularly most military ammunition), firing with a suppressor and standard ammunition and for a suppressed rifle with hotter loads.
The QPQ Isonited® barrels are turned in-house from high-quality blanks. PSW has chosen to use a heavier profile, for durability and accuracy, even at the trade-off of slightly more weight. The button rifling is given a 1:10 twist.
Using a nickel-Teflon coating on the piston bolt and bolt carrier increases durability and reduces friction. Although the bolt carrier group has greater mass than is used by some other manufacturers, the advantages are that it extends the dwell time and delay unlock, which in turn allows chamber pressures to be reduced for easier extraction. PWS has designed the parts with gaps and channels on the surface, which makes for fewer points of contact for reduced friction and build-up of debris.
Using a single billet of 6061 aircraft-grade aluminum, also coated in nickel-Teflon, the buffer tube is also fluted to carry away debris. The tube also has 4 drain holes to increase water drainage. This is only part of the buffer system design: PWS has also eliminated the castle nut, using instead an indexing screw and plate system to ensure a secure fit and precise alignment. They have also featured a lip that extends into the receiver to support of the carrier to compensate for carrier tilt.
The lightweight, free-floating handguard is also 6061 aluminum and has a KeyMod interface for accessory placement. It includes both a top Picatinny rail and two 2-inch Picatinny rail sections, plus integral sling swivel attachment points, at the front and back. All the furniture – grip, stock and magazines – are from Magpul.
If you’re looking for a piston-driven AR in .308, the in-stock PWS MK220 rifle provides the opportunity to buy a precision-made firearm in this configuration. It’s neither an AK-47 and it’s not an AR-15, although it evolved from both.