Tactical World - - Contents - By Gus Alonzo, Jr.

De­con­struct­ing a pis­ton-driven AR plat­form.

If you’ve paid any at­ten­tion to the tac­ti­cal rifle in­dus­try in the last decade, you may be fa­mil­iar with the con­tin­u­ing de­bates over which fea­tures pro­vide an op­ti­mal, all-pur­pose plat­form that ex­cels in ever sce­nario: self-de­fense, com­pe­ti­tion, hunt­ing or just hav­ing fun at the range.

In­dus­try pro­fes­sion­als and week­end war­riors alike de­lib­er­ate ad nau­seam on things like ideal car­tridge, bar­rel length, twist ra­tio, and other specs that may or may not trans­late into vary­ing de­grees of re­li­a­bil­ity, ac­cu­racy, re­coil con­trol and ver­sa­til­ity. You know that what they say about opin­ions: ev­ery­one has one.

Prob­a­bly the most hotly con­tested de­bates that have risen to the top of the list are di­rect im­pinge­ment AR ver­sus a gas pis­ton AK. Many read­ers may have al­ready heard both sides and set­tled on a pre­ferred plat­form, but what if I were to tell you that you didn’t have to choose one or the other and that you can en­joy the ben­e­fits of both sys­tems in one plat­form? In re­sponse to this ques­tion, the pis­ton-driven AR plat­form was born. A pis­ton-driven gas op­er­at­ing ac­tion in an AR up­per is not ex­actly a new con­cept. Sev­eral big play­ers in the tac­ti­cal rifle mar­ket have uti­lized this de­sign to great suc­cess, in­clud­ing LWRC, Bar­rett, as well as Heck­ler and Koch. It is my hope that I can pro­vide a com­pre­hen­sive un­der­stand­ing of what the ob­sta­cles are in cre­at­ing a pis­ton-driven AR and to an­a­lyze the unique ap­proach Pri­mary Weapon Sys­tems adopted when cre­at­ing the PWS MK2MOD1 .308 Match.

This in­for­ma­tion should clar­ify the process of choos­ing a pis­ton-driven AR plat­form. Be­fore get­ting into the de­tails, how­ever, it’s im­por­tant to first un­der­stand what fun­da­men­tally sep­a­rates di­rect im­pinge­ment plat­forms from their pis­ton driven cousins.


I hope you all can tread wa­ter, be­cause we are about to get neck deep into what en­gi­neers may call “small talk” at a party, so just bear with me! Eu­gene Stoner, fa­ther of the mod­ern sport­ing rifle, pop­u­lar­ized the di­rect im­pinge­ment (DI) sys­tem in the mid-1950s with his orig­i­nal Ar­malite mil­i­tary ri­fles. These ri­fles even­tu­ally per­me­ated the civil­ian firearms mar­ket and still con­tinue to dom­i­nate the AR mar­ket.

Throw­ing a rock down range be­gins with the pull of the trig­ger, which re­leases the ham­mer, ig­nit­ing the car­tridge, dis­charg­ing high-pres­sure gas, and push­ing the bul­let through the bar­rel. About three quar­ters of the way down the bar­rel, the bul­let will pass the gas port, where a por­tion of the high-pres­sure gas is redi­rected into the gas tube. This gas ex­pels its force di­rectly onto the bolt car­rier key on the bolt car­rier group (BCG) lo­cated in the up­per re­ceiver.

It is at this point an ar­gu­ment has been made that hav­ing these gases ex­pelled di­rectly into the up­per re­ceiver will re­sult in car­bon buildup of the BCG and other in­ter­nal com­po­nents. The con­stant ex­po­sure to car­bon re­quires pe­ri­odic clean­ing and there­fore re­li­a­bil­ity is­sues. The force the gas ex­erts on the bolt car­rier key pushes


the BCG rear­ward, eject­ing the spent cas­ing and redi­rect­ing it­self forward uti­liz­ing the force gen­er­ated by the buf­fer spring; this move­ment also cham­bers the next round. A key de­tail to remember about the DI sys­tem is that all the com­po­nents driv­ing the op­er­a­tion of the rifle are trav­el­ing in one straight line, re­duc­ing re­coil and re­sult­ing in im­proved ac­cu­racy.

The tem­plate used for the pis­ton­driven AR was grounded off the highly-re­spected AK-47 de­signed by Mikhail Kalash­nikov. Man­u­fac­tur­ers have taken dif­fer­ent ap­proaches in­cor­po­rat­ing the pis­ton driven sys­tem within the up­per re­ceiver of an AR, but the over­all op­er­a­tion is very sim­i­lar. The high-pres­sure gas ex­pelled from the car­tridge is di­rected down the bar­rel un­til it goes through the gas port into the pis­ton cham­ber where the gas ex­pands, driv­ing the pis­ton rear­ward un­til it im­pacts the car­rier block lo­cated on the top­side of the BCG. The ad­van­tage here is that the car­bon con­tain­ing gas is not ex­pelled di­rectly into the up­per re­ceiver; there­fore, it re­quires less clean­ing and main­te­nance.

Once the pis­ton drives the BCG rear­ward, the case is ejected and the bolt is driven forward by the buf­fer spring cham­ber­ing the next round. Un­like the DI sys­tem, the forces driv­ing the op­er­a­tion are not on the same hor­i­zon­tal axis, due to the lo­ca­tion of the pis­ton sys­tem be­ing above the bar­rel. As a con­se­quence, a phe­nom­e­non known as car­rier tilt oc­curs, in­creas­ing re­coil and wear. It takes place when the BCG is driven rear­ward and down­ward caused by the im­pact of the pis­ton on the top side of the bolt car­rier. This ad­verse move­ment will in­crease wear on the bot­tom of the car­rier, the up­per re­ceiver and the buf­fer tube.

With the fun­da­men­tals out of the way, check the side­bar for the ad­van­tages and dis­ad­van­tages of each sys­tem.


The PWS MK2MOD1 is available in a va­ri­ety of con­fig­u­ra­tions, available in bar­rel lengths of 14, 16 and 20 inches. The bar­rel has a 1:10 twist with cal­iber op­tions of .308 Match or 6.5 Creed­moor. They are man­u­fac­tured in-house through a but­ton ri­fling process. An isonite QPQ treat­ment en­hances cor­ro­sion re­sis­tance and in­creases hard­ness. The MOD FSC muz­zle de­vice is af­fixed to the bar­rel to aid in muz­zle con­trol and flash sup­pres­sion. A four-po­si­tion ad­justable gas block gives the op­er­a­tor the abil­ity to match the per­for­mance of the rifle to the am­mu­ni­tion and con­fig­u­ra­tion of choice. The po­si­tions are pre-set for stan­dard am­mu­ni­tion, hot­ter loads,

sup­pressed us­ing stan­dard am­mu­ni­tion and sup­pressed us­ing hot­ter loads. The MK2MOD1 is out­fit­ted with a free-float­ing hand­guard in­cor­po­rat­ing a full-length Pi­catinny top rail ma­chined from air­craft grade 6061 alu­minum. A Key­mod in­ter­face re­duces over­all weight, while pro­vid­ing the real es­tate for any at­tach­ment your heart de­sires.

The up­per and lower re­ceiver are Cnc-ma­chined from mil­spec 7075 alu­minum forg­ings and then hard­coat an­odized type III for cor­ro­sion and wear re­sis­tance. To round out the pack­age, the MK2 comes ready to go with a BCM Gun­fighter charg­ing han­dle.

The bolt car­rier group is ma­chined from tool steel, and then treated with nickel-te­flon to re­duce fric­tion and in­crease dura­bil­ity. The bolt car­rier was de­signed to min­i­mize con­tact points by cre­at­ing gaps and chan­nels be­tween the skid pads to de­crease any metal on metal wear that could be caused by nor­mal op­er­a­tion and car­rier tilt. This is yet an­other sub­tle, but very im­por­tant, fea­ture that di­rectly ad­dresses the is­sues of wear caused by car­rier tilt as seen on other pis­ton ARS.

In con­cert with the mod­i­fied BCG, PWS has in­tro­duced a car­rier tilt fea­ture onto the buf­fer to negate the ef­fects of car­rier tilt. The buf­fer tube was mod­i­fied by the ad­di­tion of a small lip ex­tend­ing into the lower re­ceiver to sup­port the down­ward forces cre­ated by BCG dur­ing op­er­a­tion.


The suc­cess of the MK2MOD1 re­sides in the de­tails the en­gi­neers uti­lized to in­cor­po­rate ad­van­tages of both the AK47 and the AR-15, in­no­vat­ing a de­sign of their own. The en­gi­neer­ing in­ge­nu­ity and the back­bone of the MK2 plat­form cen­ters on the long stroke pis­ton de­sign. This was no easy feat to tackle.

Dean Sylvester, pres­i­dent of PWS, ex­pressed that the big­gest chal­lenge his team faced was fit­ting the dual head pis­ton and op rod, which is at­tached to the car­rier into the space nor­mally used for a gas tube in a di­rect im­pinge­ment rifle. Al­ways keep­ing the end user in mind, they did not see rais­ing the sight­ing plat­form over a quar­ter inch like oth­ers in the mar­ket to be an ac­cept­able so­lu­tion to the prob­lem. Fur­ther­more, he adds that de­sign­ing around the space al­lowed has def­i­nitely been one of the fac­tors that has taken the most en­gi­neer­ing ef­fort.

What fea­tures does PWS bring to the ta­ble that are not found in other pis­ton driven ARS? Who bet­ter to answer this ques­tion than Sylvester? He said other de­signs on the mar­ket use a sys­tem where the op rod im­pacts the car­rier block to pro­vide the en­ergy to cycle the weapon.

“Key dif­fer­ence be­tween the PWS

sys­tem and oth­ers is that the op rod is at­tached to the car­rier and the pis­ton is at­tached to the op rod,” he con­tin­ued. “This pro­vided a gas to metal in­ter­ac­tion much the way a di­rect im­pinge­ment rifle op­er­ates, giv­ing the weapon a more con­trol­lable feel and less im­pact wear to the key parts over time.”

To add to Sylvester’s state­ment, the re­coil felt from a one-piece de­sign is smoother due to its push­ing ac­tion ver­sus the snappy move­ment gen­er­ated by a high-speed metal on metal im­pact tra­di­tion­ally seen in a two-piece de­sign where the pis­ton slams against the BCG. Be­cause the pis­ton is a two-piece de­sign, it al­lows the MK2MOD1 to ac­cept a charg­ing han­dle. There is a hole lo­cated on the muz­zle end of the charg­ing han­dle where the op­er­at­ing rod is passed through and then con­nected to the dual head pis­ton form­ing the long stroke pis­ton.


I was ex­cited to take my new girl­friend out on our first date at the range. The 16-inch PWS MK2 was my model of choice, equipped with U.S. Op­tics MR 10MIL 1.8-10x scope with ADM Re­con 20 MOA scope mount. The scope added some no­tice­able weight to the rifle, but the rifle it­self was rel­a­tively light and very ma­neu­ver­able. The qual­ity of the U.S. Op­tics MR was su­perb, as you would ex­pect, the glass was crys­tal clear, quick to zero and felt very solid. Be­sides be­ing a fan of an il­lu­mi­nated ret­i­cle, I was very fond of the EREK el­e­va­tion ad­just­ment drum that al­lows the user to make quick, large ad­just­ments short­en­ing the time spent ze­ro­ing the scope.

I could in­stantly tell that the rifle is made with a lot of care and pre­ci­sion. Ev­ery­thing is crisp and clean, with seam­less tran­si­tions be­tween the up­per and lower re­ceiver. Break­ing down the rifle for main­te­nance is very sim­ple and in­tu­itive. Dis­con­nect­ing the op rod and the dual head pis­ton is a cinch, but it takes a lit­tle get­ting used to as one might ex­pect. I shot from the seated po­si­tion on a bench rest, us­ing shoot­ing bags for sta­bi­liza­tion. The ammo used for test­ing was Fed­eral Pre­mium 168-grain Sierra Matchk­ing BTHP, Winch­ester Match 168-grain and Go­rilla 175-grain Sierra Matchk­ing, which was pro­vided by the man­u­fac­tur­ers.

When push comes to shove, this rifle is very ac­cu­rate. My three-shot group­ings at 100 yards re­sulted in a .76-inch group us­ing Winch­ester, 1.062 inches us­ing Fed­eral and 1.125 inches us­ing Go­rilla.

I achieved some sub MOA groups with all brands of ammo dur­ing my time on the range and was con­sis­tently able to shoot around 1-inch groups. The MOD FSC muz­zle de­vice per­formed well, but

it is no­tice­ably louder than oth­ers in its class. Re­coil was very man­age­able when per­form­ing rapid-fire shots; I was able to get back onto tar­get quickly with fol­low up shots and put ev­ery­thing on a 12x13-inch tar­get at 100 yards with very de­sir­able group­ings.

The trig­ger is one of the smoothest, most crisp trig­gers I have shot and has lit­tle to no creep. Trig­ger pull was tested us­ing the Ly­man elec­tronic trig­ger pull gauge, av­er­ag­ing around 7 pounds, 7 ounces.


You may feel like you de­serve some col­lege cred­its by the end of this ar­ti­cle, but these are ques­tions and answers be­ing dis­cussed in fo­rums by other like­minded en­thu­si­asts in the quest of choos­ing the right AR pis­ton plat­form.

You can say that I am fan of the PWS MK216. I am a be­liever in the de­sign con­cept and feel that it was well ex­e­cuted with a spe­cial at­ten­tion to de­tail that can only be ex­pected com­ing from PWS. Af­ter hun­dreds of rounds sent down range, it per­formed with­out a sin­gle hic­cup and de­liv­ered con­sis­tent, pre­cise shots.

My one cri­tique is that for a rifle in this price range I would ex­pect PWS to in­cor­po­rate a com­pletely am­bidex­trous re­ceiver, in­clud­ing an ambi charg­ing han­dle. With the MSRP of $2,200, you are get­ting a best in class rifle. Don’t get me wrong, two grand is not light on the wal­let, but in my eyes, it is money well spent con­sid­er­ing the va­ri­ety of ap­pli­ca­tions this rifle will af­ford you.

If any of you have caught the pis­ton AR bug, like I have, do your re­search and choose wisely. A good de­sign makes all the dif­fer­ence. TW

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