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Direct Impingement or Piston

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Old 02-16-2016, 06:26 PM   #1
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Default Direct Impingement or Piston

First off let me say I know nothing and I mean nothing about these rifles. With that being said-

I have been doing a lot of looking and reading but for the life of me I can't figure out what all the numbers and letters mean, 8L, 2T, 5X or what have you. What in the world do they all mean?
Also, what does direct impingement or piston mean?
I have read the the piston operated rifles run cleaner and direct the gases away from the shooter better. Is this the only reason to buy piston operated?

I am a lefty rifle shooter so, I have been looking at the Stag Arms line of rifles. Can anyone recommend a AR15 rifle that I could start out with?

Dang this stff is confusing...
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Old 02-16-2016, 09:17 PM   #2
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You'll be an expert in no time. Don't have any AR models, just a Mini-14. My nephew spent the extra buck on a piston rifle and swears by it - this from an Infantry guy who is well aware of cleaning M16s and M4s. He was able to clean his AR faster than I could my M1 Carbine...If nothing else, the cleaning time leans toward the piston type, IMHO.
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Old 02-16-2016, 09:20 PM   #3
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DI ar-15 are cheaper, but they tend to get dirty quicker, piston are more expensive but stay cleaner.
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Old 02-16-2016, 09:21 PM   #4
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Direct impingement has been used on all AR-10, AR-15 M-16 rifles for the last 50 years.
The military use these rifles in burst fire and full auto.

"Pistons" are more parts to break, increase cost and as far as I can see, are nothing more than an advertising gimmick.
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Old 02-17-2016, 01:20 AM   #5
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With piston guns the gas exits sooner and the gasses are hotter when they leave the gun. The DI guns get hot faster as the gasses stay in the gun longer, which only matters if you plan to shoot a lot of rounds quickly.
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Old 02-17-2016, 01:41 AM   #6
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Originally Posted by Caribou View Post
With piston guns the gas exits sooner and the gasses are hotter when they leave the gun. The DI guns get hot faster as the gasses stay in the gun longer, which only matters if you plan to shoot a lot of rounds quickly.
not completely true. the pistons in piston ar15 are the weak point and under extended firing it is also the fail point as the piston does not have the mass to distribute the heat. the piston is also typically a jam point in such guns which tend to be poorly designed. as the piston heats under extended use the parts stop working. allowing the piston to cool is why m240 m249 and m60 machine guns come with spare barrels.

di guns will run longer than piston guns.

the above applies to almost all ar15 with conversion pistons. some guns like tge sig mcx scar16/17 and a few others designed to be piston guns from the ground up do not have such issues

if your buying an "ar15" stick with di. if you want a piston system buy one designed to be a piston system and not a frankensteinlike abortion.

where the gas comes out is only important when running a suppressor. di guns are simply miserable to tun suppressed. i have a number of suppressors but i do not use one on my di ar15.

as for dirt. clean the piston clean the bcg your choice. the gunk goes some where, your choice of what you want to clean. it does not just automagic itself away into the ether

you cannot carry enough ammo to stop a di or piston ar15. too heavy. piston ar15 offer no advantage over a di driven ar15. piston ar15 tend to be nose heavy and heavier guns in general
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Old 02-17-2016, 01:45 AM   #7
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Originally Posted by gunman41mag View Post
DI ar-15 are cheaper, but they tend to get dirty quicker, piston are more expensive but stay cleaner.
and, a little bit cooler.

(stag model 8 owner)
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Old 02-17-2016, 07:53 AM   #8
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Like Jon said, DI is how the gun was designed. There is nothing wrong with it. If you want a piston gun, buy a piston design (SCAR, AK, etc).

DI guns do NOT malfunction from fouling. They will eventually shoot themselves dry. Add a few drops of oils and they work again.

Just as the 1911 was designed as a single action gun, modifying it to a double action gun (Colt Double Eagle) is an abortion that will not work as originally designed. Trying to get it to do something other than the original design is rarely a good idea.
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Old 02-17-2016, 05:19 PM   #9
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It was going to post a myth vs fact thread on the DI vs. piston issue, since I have probably built more of the latter than most on the forum, but since it is being brought up now, I will go ahead and do it now:

Myth: DI is a very dirty means of cycling the M16/AR15 platform of weapons, and after a few hundred rounds, you basically end up with a bolt action weapon until you clean it, while your enemy is still shooting his AK 47 at you.
Fact: When they leave the factory, a well built DI rifle or carbine will be able to shoot several thousand rounds between cleanings. You just have to lube it periodically, depending on how hot you are getting it.
Origin: Many military troops have armorers who have a DILLIGAS attitude with regards to their level of maintenance on the weapons, and some civilian manufacturers have the same mentality towards building them. Both can leave a sour taste in the mouth in the mouth for the end user.

Myth: Piston AR's are much more expensive than DI AR's.
Fact: If you buy the weapon with the piston already in, or convert your existing AR to piston, they add from $250 and up to the cost of your AR. If you build the weapon or the upper yourself using a stripped upper and stripped barrel, you can have on for about $30 to $50 more than a DI configured AR.

Myth: Piston AR's are much heavier than DI AR's
Fact: The piston will always have more weight, but there are some who only increase the weight by about the equivalent of a couple of rounds of ammo.
Origin: Some manufacturers that manufacture piston AR's exclusively have them at 1-2 pounds over the weight of a similar DI AR. The heaviest is the HK 416, which is 2 1/2 pounds over the weight of an M16.

Myth: Piston AR's fire under water
Fact: Never, ever try this. No firearm is designed to fire under water. Period.
Origin: Manufacturers push their weapons to the extreme because their intended end user may be a Navy Seal or Delta Force operator in a mission to save lives, and may have to swim to the target, fire the suppressed weapon, and submerge again due to the mission parameters. Any time you submerge a firearm, dry it out before firing. Most weapons can take a degree of moisture in them (you can't call off a war because of rain), but submerging any firearm gas system is dangerous, and should be avoided unless the risk of loss of life is greater if you don't.

A few true facts about piston AR's: 1. They have a better track record for firing steel jacket Russian ammo than DI's, although I know of a couple of DI owners who will attest they don't have any more FTF's out of the Russkie ammo than brass. 2. Because of increased recoil, they can use the bumpfire stocks better than DI guns. 3. Depending on the piston itself, they can put the carbon in easier to get to places. 4. Piston AR's are LESS prone to cookoffs due to hot gas not going inside the bolt carrier. They can and do still happen, just less frequently.

This is my perspective, having owned several of both types of AR's. Very few of the shooters who post really bad opinions of piston AR's do so from experience, but rather regurgitation of something they read on the net. Piston AR's have their uses, but I don't recommend them for beginners.
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Last edited by fsted2a; 02-17-2016 at 07:35 PM.
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Old 02-17-2016, 07:01 PM   #10
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There's no discernible advantage to external pistons in AR-15's for civilian use and in military use the guns are run so hard that the barrel becomes the limiting factor in how many rounds you can put down range. That'd be why the M4A1's use full heavy profile barrels instead of M4 profile barrels.

The point at which an internal piston AR with a properly built gas system would fail from overheating is also the point at which severe barrel damage occurs. Colt and many other manufacturers have extensively tested their products. Our military has tested their products in ways that small arms designers wouldn't even dream up.

If you keep an AR bolt lubricated with a high temperature grease, a properly built internal piston or external piston AR will keep firing until you run out of ammo or melt the barrel, whichever comes first.


Is your AR fully automatic and do you intend to run a half case (M4 profile barrel) or full case (M4A1 profile barrel) through your carbine in roughly two to five minutes?

If not, then buy a semi-automatic internal piston (direct impingement) AR that you like best. The US military has already pondered your conundrum and it's still busily issuing DI AR's. So far, nothing substantially better has come along at a reasonable price that can withstand a firing schedule that melts 4150.
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