Best AR-15 Piston Uppers & Conversion Kits [2019]

It seems like everyone and their mom makes a piston-driven AR-15 these days.  And with good reason.

Piston-driven guns run cleaner and cooler than a standard direct impingement (DI) system, making them a big draw for high volume shooters such as those who shoot competitively like in 3-Gun.

PWS Piston Cutaway
PWS Piston Cutaway

They also just make all around great guns if you want to get into AR-15’s a bit more and try something different.

Who knows?  You might just fall in love.

PWS MK1 Mod 2 Complete
PWS MK1 Mod 2 Complete

Unfortunately, not everyone has the kind of cash required to pick up a whole new gun just to give this fancy schmancy piston thing a try…which is totally understandable.

So what do we do?

Adams Arms P-Series Piston Kit
Adams Arms P-Series Piston Kit

One option is to simply switch out the upper receiver with a piston-enabled upper receiver and be on your merry way.  Or you can use a conversion kit to change your DI system into a piston one.  We’ll cover our top picks in each!

If you can’t wait…here are our picks for best AR-15 piston uppers and conversion kits:

Best Budget Piston UpperStag Arms Model 8HSC
Best Bang-For-Your-Buck UpperAdams Arms P1/P2/P3 Series
Upgrade Piston UpperPWS MK1, MOD 2
Super Upgrade OptionBarrett REC7 Gen II
(Previous) Editor's PickAdams Arms Conversion Kit (P Series)
Newer Piston KitSuperlative Arms Piston Kit

I’ve used all of them…but first let’s go over why you’d even want a piston AR in the first place.

Table of Contents


Getting a Piston AR-15 Without Breaking the Bank

Well, if you’re anything like me when I started out, you probably already have a cheaper (but no less awesome) direct-impingement AR-15 laying around.  

Aero MC-15M
Aero MC-15M

If that’s the case, then you can just pop your current upper receiver off, and slap on a new piston-driven upper.  It’ll work just fine with your lower.

AR-15 Lower Receiver
AR-15 Lower Receiver

If you don’t already have an AR-15, I highly recommend picking up a DI gun first before springing for a much more expensive piston-driven one.  If you need help choosing your first AR, I also recommend giving our updated AR-15 Buyer’s Guide a look.

So, what piston-driven upper should you get?  How do you choose?

Well, I’ve tried out way, way, waaaay too many of these things (just ask my fiancé ) so I’ve got a list of what I think are the best piston uppers currently out there.  

Why Choose a Piston AR-15

This is going to be a bit of a Coke vs. Pepsi, Ford vs. Chevy, Superman vs. Goku type of deal, so it’s important to understand right away that a lot of this is going to come down to personal preference.

Superman vs Goku
Superman vs Goku

However, there are some tangible benefits to choosing a piston AR.  You’ll have to decide if they’re worth it.

Also, the correct choices are Coke, Ford, and Goku, and you can tell me why I’m wrong in the comments.

Benefit #1: No gas blowback into the breech

With a piston AR, you don’t get the gas and powder fouling into the breech like you do with a standard DI firearm.  Instead, you get just a small amount of powder fouling in a small, contained area around the piston itself.

Why does that matter?

Well, with a standard AR-15, you get exhaust gases and powder, and all that crud and gunk that comes from firing a gun dumped right back into the breech.

Direct Inpingement Gif
Direct Impingement Gif

This means all that junk winds up in your lower, which makes for a much dirtier firearm, and much more frequent cleanings if you don’t want your gun to start having some issues.

So basically, a direct impingement gun shits where it eats, which is…not ideal.

Piston AR Animation, Adams Arms
Piston AR Animation, Adams Arms

Meanwhile, a piston AR funnels all that gas and powder fouling into a contained area (face of the piston), which makes a piston AR a much easier beast to clean and maintain.

PWS Piston Cutaway, Piston Highlight
PWS Piston Cutaway, Piston Highlight

Benefit #2: Heat Displacement

You know how other guns get hot when you shoot them?  Yeah, same thing happens with AR-15s.

However, for really high volume shooters, like say if you have a legal fully automatic lower, or you shoot competitively, you can keep your gun a little bit cooler by swapping to a piston system.

AR-15 Bacon Cookoff, RescueHumor1
AR-15 Bacon Cookoff, RescueHumor1

This is because, again, you’re not dumping hot, nasty gases into the main body of your firearm anymore and are instead keeping things contained.

Downsides to Piston AR-15

Now, it wouldn’t be much of a choice if that was the whole story.  There are some downsides to the piston lifestyle that I want to go over as well.

First, they’re pricey.

As in, some of these upper receivers alone cost as much as fully-kitted direct impingement gun, pricey.  But, for some people that’s okay.

Khajit Has Wares If You Have Coin
Khajit Has Wares If You Have Coin

Second…parts can be a bear to find and match up properly.  Piston systems aren’t necessarily standardized like direct impingement systems.  So if you go with one manufacturer, you’re stuck with them.

So make sure you choose one that has a good track record and will be around in the future.

Lastly, since there’s more metal stuff in the front, the gun can become front heavy.  I haven’t found this to be a serious hindrance but it’s a little different from your normal DI gun.

So, with all that being said, let’s get into some of the best piston uppers and upper conversion kits on the market.

Best AR-15 Piston Uppers

1. Stag Arms Model 8HSC

Best Budget Piston Upper
663 at Stag Arms

Prices accurate at time of writing


  • Barrel Material: 16″ 4140 steel, chrome lined, government profile, manganese phosphate coating
  • 1:9 Twist
  • Muzzle Thread Pattern: 1/2″-28
  • Bolt Carrier: 1-Piece w/Integral Piston Strike Face
  • Samson Star-C free float quad rail

The Stag Arms Model 8HSC is a great budget piston upper and comes with a very usable quad rail.

Plus, you can configure your upper according to your states’ laws (hi CA, NJ, CT, MD).

Keep in mind that this is upper has a carbine gas system.

2. Adams Arms Tactical P1/P2/P3 Upper (Mid-Length)

Best Bang-for-Buck Piston Upper
600 at Brownells

Prices accurate at time of writing


  • 5.56mm NATO 16″
  • Barrel Material: Chrome Moly Steel
  • 1:7 Twist
  • Muzzle: Pinned A2 Flash Hider
  • Barrel Finish: Melonite / Black Nitride
  • Bolt Carrier: Black Oxide Coated Steel
  • Free Floated Aluminum Forearm
  • M4 Feed Ramps

The Tactical P1/P2/P3 series from Adams Arms is the industry standard for a solid, reliable, piston upper.

Confused on the three options? Basically, P1 is the basic entry tier upper, P2 is more standard, and P3 is upper tier designed for competition or duty carry.

All three are great, but P2/P3 have some extra features such as adjustable gas blocks and such. They also have a higher price to match.

It comes with a very nice aluminum handguard that has a full flat-top rail that connects seamlessly with the rail on top of the upper giving you the ability to mount every kind of sight, scope, laser, can opener, and tactical assault coffee mug your heart desires.

Battle Mug
Battle Mug

Seriously, the rails on this thing (the gun, not the coffee cup) are incredibly functional, and (re)movable so you can place them where you want, or lose them completely for a more high-speed, low-drag look.

Beyond that, the upper functions flawlessly with proper maintenance when used in conjunction with a quality mil-spec lower. It is a bit more expensive than the Stag, but it’s worth it if you have the cash, or want to upgrade a little bit.

Or if you need four lights and a laser to feel safe.  Hey, no judgment here.

Mall Ninja AR-15
Mall Ninja AR-15

Ok…a little judgement.

Another great thing is the mid-length gas system.  I think it is the best of both worlds and splits the difference between the shorter carbine and longer rifle uppers, which makes this a bit of a Goldilocks upper…least for my purposes.

Midlength Shooting Target
Midlength Shooting Target

I think the midlength platform is probably the best place to start out for most newbies, and it’s a great starting point for those who just want to try something different as well.

You get a longer forearm than with a carbine length, which makes for an easier shooting experience, but you don’t have too much barrel slowing you down as you do with a rifle length gun.

This is especially an issue when you’re trying to move during a competitive match, a home-defense scenario, or in the confines of a deer stand.

Many a deer has been spooked by a hunter banging their barrel against something in the stand.  No that’s not a euphemism.

3. PWS MK1, MOD 2


  • Barrel Length – 14.5″ (Pinned to 16″)
  • Overall Length – 23.4″
  • Gas Piston System – Carbine
  • Barrel Twist – 1:8
  • Caliber- .223 Wylde
  • PicMod Rail System Handguard
  • Forged upper
  • Adjustable Gas Block

If you have the money and want the next level up piston upper, get the PWS MK1, Mod 2

If you don’t have the money and want the very best possible piston upper, get this one anyway.

If the fecal matter has hit the fan, the apocalypse has come, and you’re digging through Rainier’s warehouse to try and build something that will help you survive the wasteland that our world has become, get this one.

Rick Wishes He Had The MK1 Mod 2
Rick Wishes He Had The MK1 Mod 2

Seriously, all kidding aside, Primary Weapon Systems has made what is, in my mind, one of the very best drop-in, no-fuss, shoot-all-day, piston-driven upper on the market today.

This upper has all the features you could possibly want, including the unabashedly cool PicMod System that allows you to use Keymod and Picatinny attachments.

It also has a forged upper receiver, making it stronger and yet lighter than comparable billet receivers.

Finally, it has an easy-to-adjust, low-profile gas block that will allow you to dial down the felt recoil of the gun to get the best possible accuracy and performance possible. This is also key if you’re going to run a suppressor on your gun.

PWS MK1 Mod 2 Complete
PWS MK1 Mod 2 Complete

It’s sitting on top of one of my competition rifles right now, and I have no plans on taking it off anytime soon.

Readers' Ratings

4.98/5 (203)

Your Rating?

4. Barrett REC7 Gen II

Super Upgrade Option
1850 at Euro Optic

Prices accurate at time of writing


  • Barrel Length – 16″
  • Overall Length – 33.25″
  • Gas Piston System – Carbine
  • Barrel Twist – 1:7
  • Caliber- 5.56 NATO
  • Barrett Enhanced Rail with KeyMod
  • Adjustable Gas Block

If you want one of the most expensive piston uppers out there right now…check out the REC7.

While most piston guns don’t really sell their uppers separately…you’re in luck with Barrett.

However, I’d rather have the mid-length gas system and MLOK rails opposed to the carbine gas system and KeyMod.

AR-15 Piston Conversion Kits

Adams Arms Adjustable Gas Piston Kit

Now, if you don’t want to buy a whole new upper, there are also a number of gas conversion kits out there that will take your DI gun and convert it to a piston system.

I’ve tried several.

I hated all of them but this one.

The Adams Arms Piston Conversion Kit will let you convert your DI upper into a piston upper with a minimum amount of fuss. I love it, and I’ve used two of them now and I couldn’t be happier. I can’t really recommend any other kits in this category because:

A) I haven’t found another one that I like, and…

B) There aren’t really any other ones that are worth your time.

This kit comes with the same adjustable gas block that comes in the Adams Arms upper I told you to buy just a minute ago, and it is altogether the best kit to toss in a mid-length rifle.  They also have kits for rifle and carbine versions as well.

Editor's Pick Piston Kit
250 at Brownells

Prices accurate at time of writing

*Update* Adam’s Arms has their new P-Series piston kit which is MUCH lighter than their previous kit.

I had the chance to shoot it recently and I couldn’t tell that it was a piston conversion since it wasn’t front heavy at all.  My new recommendation for the best adjustable gas piston kit.

Superlative Arms Adjustable Piston Kit

I love Superlative Arm’s adjustable gas block.  In fact…it’s the only one that I use now.  See the full review.

Installed Superlative Adjustable Gas Block on Faxon 14.5 Pencil
Installed Superlative Adjustable Gas Block on Faxon 14.5 Pencil

So I can’t wait to try out their piston system due to their same rave reviews…and how you can easily find the different gas lengths PLUS pencil barrel size in stock.

Stay tuned!

Final Thoughts

That about does it for this one, guys and gals.

Those are my recommendations for choosing a piston based AR-15 upper.

Want a normal DI upper? We’ve got our favorites in our Best AR-15 Upper Assemblies for the Money guide!

Do you use a piston upper? Did you switch back to DI? Let me know in the comments!

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Bert PowersEric Lautzjareth belangerCole WarrenSteveB Recent comment authors
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Bert Powers
Bert Powers

I prefer shooting and cleaning the gas piston over the direct impingement. The gas piston has a little less recoil too.

Eric Lautz
Eric Lautz

I gotta say this article sucks! Its full of errors, one big one is that the PWS FULL RIFLE SHOWING IS THE MK1 MOD1, NOT MK1 MOD2. The MOD1, has a forward asisst the MOD2 does NOT. ALSO almost every RIFLE maker that makes a piston rifle WILL SELL THIER UPPERS SEPARATELY, So not sure where that came from. Also Barrets upper is not one of the most expensive out there, its on PAR with all the other HIGHEND piston systems, you have just picked it to compare to all the bottom end ones. PWS IS MIDDLE GROUND, BUT the… Read more »

Cole Warren
Cole Warren

Can anyone tell me if these conversion kits will work with my current free float rail system? Its a Leapers UTG super slim 13 inch

jareth belanger
jareth belanger

Both Superlative Arms and Adams Arms have a diagram for their block dimensions, so look that up and measure your rail. Also on the adams arms you might have to notch the barrel nut on the rail system for the recoil spring so it will sit at milspec length away from the receiver. That being said I have shot both and the superlative is the easier to fit one, but the adams arms is more robust. If you are going to be doing fullauto or binary mag dumps, go with the adams. If you just want a piston for cleaning… Read more »


I got the same pencil barrel setup for a light weight build (coming from a 16″ HBAR). I works great with an Griffin Opitmus on the end – with one huge downside. The 1 pound can causes the barrel to bend 0.006″ (6 thousanths) at the gas block and equates to a 3″ lower bullet strike at 25 yards. So I will run it unsupressed and get the 11.5″ mid-length gas Faxon Gunner barrel that is stiff and designed for dedicated suppressor use.


I know this is a dead thread by now, but that Superlative piston system is the best IMO. I think in another year after it’s been compared to the rest by all the gurus out there that it will rise to the top of the list. Several enlisted gear heads I know swear it’s a much better designed system than the other piston kits like apples and oranges and I agree. Maybe others will pip in and add to that testimony ?

jareth belanger
jareth belanger

I did a 10 (30rd) mag dump through it with a binary trigger and a shot timer with around 670rpm per mag and ~2 sec mag changes on a shot timer, estimated because on mag 6 it seized. I melted the recoil spring inside the housing and mushroomed the bottom of the piston in the housing because of that, had to drive the piston out with a hammer after it was pulled off the barrel. That being said, 99% of people will not torture their rifle like that. But if heat absorption is an issue. I would go adams arms… Read more »

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