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Old February 7, 2013, 10:37 PM   #1
raymondvillain
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direct impingement or gas piston for .308?

For an AR type semi-auto in .308, what are the advantages of a gas piston with action rod as opposed to a gas block and a tube to carry the gas back to the bolt? Wouldn't a piston set-up keep the action cleaner?

And for the AR platform, why isn't the gas block closer to the muzzle? I remember the M1 had the gas piston almost at the end of the barrel. The M1A (or M14) has the piston farther in, towards the receiver, but not nearly as close to the receiver as the AR platform, where it seems to be about halfway between receiver and muzzle. It seems like putting the gas block (or piston) so close to the receiver robs the bullet of much needed pressure.
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Old February 8, 2013, 08:53 AM   #2
tobnpr
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Welcome to the forum.
Try a search on this subject, DI v. Piston has been debated ad nauseum...often getting pretty heated.

But, to answer the direct question you posed:
Quote:
Wouldn't a piston set-up keep the action cleaner?
The answer to that one is beyond any type of debate, it's an absolute "yes" as the combustion gases are not blown back into the receiver.

Far as location of the gas block, it is a function of barrel length, and pressure needed to reliably cycle the action.

Don't forget that you're comparing apples and oranges, in that the M14 is cycling full-size rifle rounds with a much heavier action than the AR platform.
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Old February 8, 2013, 01:26 PM   #3
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If you ask for a recommendation next, look into the PWS MK216. Be prepared to wait though.
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Old February 8, 2013, 02:12 PM   #4
Dr Big Bird PhD
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I've only dealt with DI systems, but from I read: the platform that the gas system is on is what matters. A piston on an AK-74 will work until doomsday, whereas a piston system built into the AR platform may not be as practical or effective.
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Old February 8, 2013, 03:20 PM   #5
blfuller
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You can start with a DI and upgrade to piston with a conversion. It might get your foot in the door sooner. Then, if you shoot so much .308 that you just absolutely hate cleaning the innards of your receiver, buy a conversion.
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Old February 8, 2013, 03:38 PM   #6
10mm4ever
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A piston system runs cleaner and cooler. There's a good reason that LMG's aren't DI. The only advantage to DI is a slight saving in weight and recoil, IMO.
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Old February 8, 2013, 03:52 PM   #7
SR420
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Nothing against DI systems, I shot DI for about 25 years and never had a problem with any DI system.
That said, all of my mag/drum fed semi-auto rifles are piston driven. I dislike cleaning rifles, knowing piston
systems run cooler and cleaner appealed to me right away. I run a sound suppressor on all of my piston
rifles and they do stay surprisingly clean. .308, 7.62x39 and .223.

Go with a proven .308 piston system if you want less cleaning and/or if you plan to run a suppressor.
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Old February 8, 2013, 09:10 PM   #8
Art Eatman
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After years of reading articles in the gunzines and posts on the Internet, if reliability is a major criterion, the answer is an unequivocal, definite, categorical, absolute "either".

And that's all I know about it.
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Old February 9, 2013, 08:39 AM   #9
madcratebuilder
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Quote:
A piston system runs cleaner and cooler. There's a good reason that LMG's aren't DI. The only advantage to DI is a slight saving in weight and recoil, IMO.
You still have the same gas coming out the barrels gas port. The heat and dirt just accumulate in a different location. You are cleaning a piston and gas block instead of the BCG.

The 5.56 piston system has the bugs worked out and is a viable option. The 7.62 piston system should be as reliable. Not sure if the extra weight is worth it.

Hanging moving parts on the barrel is counterproductive to accuracy.
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Old February 10, 2013, 05:42 PM   #10
buckhorn_cortez
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Quote:
You still have the same gas coming out the barrels gas port. The heat and dirt just accumulate in a different location. You are cleaning a piston and gas block instead of the BCG.
Depends upon the design of the gas block and piston system. I have a Barrett REC7 and it is self cleaning. While that sounds about like a "perpetual motion" claim - I cleaned the piston and gas block after 5,000 rounds and only got a tiny bit of grey fouling on a cleaning cloth when I wiped them both with a gun cleaner.
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Old February 10, 2013, 06:15 PM   #11
JohnKSa
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Quote:
The only advantage to DI is a slight saving in weight and recoil, IMO.
And potentially better accuracy given that there are fewer moving parts, particularly fewer moving parts impinging on the barrel.
Quote:
Don't forget that you're comparing apples and oranges, in that the M14 is cycling full-size rifle rounds with a much heavier action than the AR platform.
Direct Gas Impingement has been around a lot longer than the .223 round.

The Swedish Ljungman (1942 design) was DI and chambered in 6.5x55 Swedish.

The Egyptian Hakim was DI, a variant of the Ljungman, chambered in 8mm Mauser.

And, last but not least, the DI, AR-10 was chambered in .308 and was developed before the AR-15.

The point is that there is absolutely nothing about the DI system that keeps it from performing well with "full-size rifle rounds".
Quote:
You can start with a DI and upgrade to piston with a conversion.
If you want a piston system, buy a system that comes directly from the factory as a piston system. The conversions can work very well, or they can be problematic depending on a number of variables, since the energy is transfered to the bolt in very different ways in the two systems.
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Old February 11, 2013, 08:01 PM   #12
Suwannee Tim
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The heat and dirt argument against DI is overstated. The heat is minimal and the dirt is minimal. Specifically when shooting my DI 5.45mm AR I get the barrel hot, hot, hot and the action is only warm. The action gets dirty but it is nothing to clean a DI gun. The DI is a brilliant design, simple, cheap and reliable. If the rifle you are interested in is available as DI you won't regret it. Except DI is not cool, not trendy. The argument changes with a suppressor but I won't address that as I have no first hand knowledge.
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Old February 12, 2013, 07:45 PM   #13
Slamfire
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If and when I get a AR-10 platform for target shooting, I am getting DI. I only shoot 88 rounds in a day in an Across the Course Match. The DI system has less moving parts and is better for target shooting.

However, one of the Army Reserve shooters I shoot with, a gentleman who has been on a number of deployments, while he target shoots with a DI, his home defense rifle is a gas piston. He says it runs cleaner and cooler.
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