Question on AR platform, DI vs piston

Have a piston pistol and rifle, find the pistol offers a unique advantage by eliminating the buffer tube. Both function well, like the ability to adjust for different loads.

I have built several piston driven uppers. I’ve never seen one actually do anything which justified the increased cost, and certainly nothing which made up for the accuracy loss and increased weight.

It is a testament to the quality of Stoner's original design that it works even with an aftermarket piston shoehorned onto the top of it.


I don't personally see any need whatsoever for the 'piston' ARs, as the problem that it is supposed to fix, isn't a problem at all.

Standard AR15s with the expanding gas piston in the bolt carrier work very well and are very reliable. They are also quite accurate with a good barrel, very modular, and light weight compared to many other similar rifles.

If you want an AR stick with the DI design, if you want a piston rifle then get one designed as a piston gun from the start. Overall, you will have less issues. As a side note, a local PD has filed a lawsuit against Adams Arms, they bought a bunch of rifles from them and they don't work as advertised.

whisler said:

I have little to no knowledge about AR platforms, but now have an opportunity to choose between 2 different rifles. One is an Adams Arms AA-15, which I know is piston driven and the other is an S & W M&P-15, which is direct impingement. I have 2 questions.

1. What are the pros and cons of the 2 different systems? (I understand the major difference but not the details)

2. Which would you choose between the brands? The S & W definitely has the nicer furniture, but that can all be changed.

My lack of knowledge comes from a lack of interest in the platform until now, due to being an old guy who prefers wood and steel and even better if it loads from the front and uses flint for ignition. But the time comes when even old guys need to catch up, so give me a hand.
Any help greatly appreciated.

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Between the two brands, the S&W is the better choice. Set up correctly, there is little practical difference between a direct gas AR (ARs do not use a direct impingement system) and a "piston" AR. However, the Adams piston AR isn't as reliable or durable as the direct gas S&W MP-15. Not because a "piston" AR is less reliable or durable- the piston driven HK 416 is tough enough for combat in harsh environments. Pass on the Adams and get the S&W.

I am a DI fan. It is less normally less expensive and a DI AR15 for the most part is lighter than a piston AR.

The reason direct gas ARs are lighter is because piston uppers usually have heavier barrels.

So basically the takeaway here is there's really nothing wrong with piston uppers, but no major advantage to them either. Get the S&W.

cfullgraf said:

In any case, the AR-15's gas system is pretty simple except for the fact that gasses are released into the action. But, it has worked pretty well for our government for the past 60 or so years.

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This is a gun shop commando fallacy that needs to die a rapid death. The gas is NOT released into the action. It is vented out the ejection port through the two holes in the side of the bolt carrier. Watch a slow motion video of the firing cycle and see for your self.

taliv said:

nothing wrong as long as you don't mind
-extra unnecessary moving parts on the end of your barrel
-more weight
-proprietary system where replacement can only be from 1 vendor
-potential decrease in accuracy
-a lot more $

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The majority of the extra weight comes from the use of heavier profile barrels. In the interest of objectivity, I looked for counters to
your other points. I got nothing.

Possibly the extra weight on an even thinner barrel would make any change in accuracy even more obvious.
Possibly it's because people that seek out piston ARs do so because they consider it a high-end, more durable rifle, and that's also also the image a heavy barrel gives them.

A heavier barrel profile handles heat better. I suspect HK used a heavier to improve reliability and durability of the 416 so they could sell the military the idea that the extra capability came from the piston design.

A thicker barrel is stiffer than its pencil profile counterpart. The extra stiffness would, in theory, be less susceptible to the reciprocating mass impacting precision.

Weigh gas block and tube.
Weigh operating rod, spring, piston if not integral, and piston gas block.

They weigh more. Nothing to do with the barrel.
Many use a heavier carrier too. Then couple that with a heavy buffer and recoil spring. Then all that slams the cartridge into the chamber so hard it can change the headspace of the brass even. But I digress, when @Varminterror says they are more inaccurate, I know he has the experience with them to make that statement. What I wonder is why they are.

I was told once the bullet is gone before the gas even enters the tube. I assume this to be false, otherwise the reciprocal mass atop the barrel would make no impact on the, impact.( ) Yet many account that it does. And I believe them. I bought them any way.

Why is it? Can it not be compensated for?(Obviously not or this wouldn’t be a question.)

I don’t mean to usurp @whisler’s thread, but this seems as good a time as any for the question...

The difference in weight between a "piston" upper and a direct gas upper with comparable barrels is just a few ounces. It might be noticeable to the shooter, but the biggest difference between the two is barrel weight.

The reason the op rod affects precision is that it can introduce a dissonant vibration and/or pressure point on the barrel while the bullet is making its way down the bore. Similar to a stock or fore end that's not free floated.

Having the buffer located near ones shoulder such as is with a standard AR has its advantages of balancing out the rifle well. And with hinged buffer plates collapsing a standard AR down has become a possibility.

There is still the downside of not being able to fire multiple times when collapsed that a piston gun gets around.

There was always too many downsides when comparing the two for me to bite on a piston AR, that and I’ve been more than impressed with all my AR’s.

I've had no issues with DI ARs at all, when you run them suppressed they get dirty a lot faster than unsuppressed, but adjustable gas blocks help with that.

As an interesting data point, the MAC YouTube channel has been testing a BCM rifle since early 2019 to see how many rounds they can shoot without re-lubing it or cleaning it before it chokes.

They just passed 6,000 rds.



Not that every rifle would go that long, but it is a testament to the well established fact that the DI system is perfectly satisfactory.

I have built a handful of ARs and here is the thing, almost all of the really big guys are making the same product made using the same small parts and forgings from one of 3 Aluminum forges. That's to say if you know what makes makers marks to look for, the name in big letters doesn't matter, and finding the best value in the bells and whistles and differences in fit and finish can be very challenging, but any name brand rifle/pistol will flat out run.

I wouldn't buy any of them new, because I know I'm going to change out half of it regardless. It's one of the very easiest platforms to put together from scratch and I STRONGLY urge anyone to do so. Nothing will give you confidence in your tools like making them yourself.

As far as DI vs piston, if you look up the military tests the DI colt M4s were sand blasted continuously for 25 hours and then fired. Collective 60k rounds. Those DI rifles experienced a less then 2% failure to function rate under those conditions and none of those were difficult to return to action. That's incredible performance under the conditions.

Now, the DI system IS inherently more accurate, however shot to shot accuracy of piston DMR and precision styled rifles is still sub MOA and with FAR less heat and grime in the action, and piston rifles (but not AR), had half the failure rate or less in the same tests, stupid reliability. There are a fair few piston rifles on the market and conversion kits too. Now a good military contractor (Colt or FN) DI rifle will run you $1100 or so, and the contractor (HK) piston rifles run you around $3000. That's not worth it to me, not even close.

On the other hand, the Ruger piston SR series is around $1500, and a lot of rifle for the price, but the Adams conversion kit is considered to be the best by the most and tics all of my boxes for features: forward gas ejection, adjustable gas block, and short stroke piston/transfer bar system (like an sks/svd) runs about $200 as an upgrade to my $3-500 pistol piece meal builds, or $500 Ruger rifle/pistols. Now THAT is my speed.

So, I urge you to build either way, but if you must buy, the Rugers are the cheapest big brand, amazing customer support and nitride everywhere that matters, machining and fit and finish make the $1100 Colts look like garbage. If you just must spend a little more the Springfield Saints are similar in quality with more bells abd whistles and nickel boron buttery triggers. Just above that, the Springfield Edge are a ridiculous bargain, built like rifles twice their price.

Best of luck.

taliv said:

in fairness, i think arguing semantics of "the gas" and "released" is misleading. yes, much of the gas that comes through the gas tube vents to atmosphere through the holes in the carrier as you described. but you could say the gas key, carrier, bolt, etc are "the action" and from the perspective of the barrel/gas tube, gasses are released into the action. of course they don't stay there forever.

the point is, they are dirty and they pass through the action, and it's totally not a problem. and there is still a lot of pressure in the barrel when the bolt unlocks and the action opens, so gas from the barrel enters the receiver from the chamber, and gas from the tube enters the receiver after the gas key retracts. and none of that is actually a problem, as long as things stay properly lubed.

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The gas bled from the barrel to operate the action is vented from out the ejection port. Most of the "stuff" found in the action (other than that deposited on the bolt tail and INSIDE the bolt carrier) are particles and gasses that come out of the case as it is extracted, and the residual gases left in the barrel that are free to flow back in to the action after both ends of it are unplugged. Same as what happens in a bolt/lever/pump/whatever other type action you can come up with.

@Demi-human - I want to be clear in my statements, as I don’t really condemn piston systems, but rather, I want folks to understand the implications of buying them, better than simply believing the advertising hype around them.

I typically mind my words to say pistons are less accurate, not necessarily “more inaccurate.” I know that is mincing words, but it is a clarification I want to make to emphasize how little difference there can be on paper. If you had a rifle shooting 3/4” with a DI set up, converting to piston wouldn’t throw you out to 2” groups, and vice versa - assuming you adjust the load for the new natural harmonics. But you do have more things happening, and more “non-barrel mass,” attached to the barrel.

Also to clarify - the gas piston MUST be in motion before the bullet leaves the barrel. Simple physics. We need the piston to move, meaning it has to accelerate, and against spring tension. So the gas pressure (force distributed over an area) has to accelerate the piston and start it moving while the bore is still pressurized - once the bullet leaves the barrel, that pressure drops to zero, meaning the force the gas exerts against the piston drops to zero, and it can no longer accelerate - move - the piston. The bore and piston “cylinder” must be under pressure to apply force on the piston, so the bullet still has to be holding pressure in the bore. Once a force is exerted, the piston accelerates - starts moving - and it can only remain moving after the bullet leaves the bore and the force/pressure subsides by its own inertia (state of movement, current velocity when the force stops acting). So it’s patently false to say “the bullet is gone before the piston starts moving.”

I would refuse to build on piston rifles if they couldn’t be made accurate, but I know it’s simpler to produce a smaller shooting rifle with a broader set of loads with a DI rifle than a piston gun.