Piston or DI for a Grendel Build?

[Klem]

Piston if you are going to fire suppressed. They are just easier with a flip of the switch and run cleaner. Other than that DI. BANG BANG!!

Not my experience unfortunately.

The crud and spit comes back past the case as it unlocks/extracts and into the receiver that way. Higher pressures for longer in the barrel mean as soon as the case no longer seals there's another path of least resistance the gas takes. The receiver ends up dirtier than unsupressed Direct Impingement. This experience from using an Adams Arms (AA) piston in a suppressed .223.

The AA also has only two functional settings; full open and half closed. Half closed is what is used for normal unsupressed. Although to be fair it doesn't feel like the action is being belted around when suppressed.

[lawndartmike]

Everything I have read or seen has DI AR15s as being more accurate than piston AR15s. Not to mention cheaper and lighter.

Any links to what you "have read and seen"?

I have an AR Stoner 16" Grendel barrel that will shoot sub MOA five shot groups at 100 yards with my reloads and the AA piston system installed. I have yet to see any real test (just a lot of post saying that they saw it somewhere) as to what accuracy we give up. Is it .125 MOA?

Putting a piston system on an AR does not automatically give it AK theoretical accuracy. Definitely there are a few ounces added to the weight and the recoil impulse seems a bit sharper than our DI rifles. My son and I shoot an average of 200 rounds through each of our ARs every month. Two are DI (5.56 and 7.62x39) and two are running AA pistons (Grendel and 5.56) and I would say they are all four great shooting and reliable. (Well, the DI 7.62x39 has some rounds that fail to ignite, but I blame that issue on crappy ammo.)

If you shoot as much as we do and clean your rifles after each outing, you will see a big benefit in time saved cleaning. With the piston system you are pretty much cleaning the rifle as quick as if it were a bolt action. There does seem to be some extra wear in the upper but then again we shoot a lot of rounds through them.

If you don't want the extra ounces and/or don't clean your rifle very often then I would say the piston system is of no real benefit for you.

Sorry for rambling, but I am really tired of people talking authoritatively about things that they have no real world experience with. I am no expert but at least I do own both and shoot them enough to recognize the pros and cons of each.

[chaps]

Piston vs. DI accuracy, both vent gas at the same time, through the same ports. Both send energy rearward to move the bolt, one blows gas the other pushes a piston (which may adjust balance if its not in line). Recoil and its effect on accuracy is more a factor of the barrel (weight, profile), buffer/spring, bcg weight, and length of system (i.e. pistol, carbine, mid, rifle), and barrel harmonics effected by bands, points. Therefore, all other things the same, accuracy should be same for cold bore single shot precision shooting. But DI is cheaper and lighter, so get a good barrel, buffer, bcg, rifle length adjustable block tuned to your load, and have at it.

Rapid shooting, the piston preserves accuracy by reducing heat between the block and chamber, especially on a lightweight profile barrel. This explains why the Marine Corps is pricing m27s (modified HK417) for 0311 - they need to be able to go from rapid fire, to controlled precision fire at variable ranges, in the same weapon as the engagement shifts, without sacrificing accuracy. Cleaner running also helps, as it saves time for maintenance - that doesn't mean DI is less reliable, just requires more maintenance time. Cooler hand guards aren't something non-infantry or 3gunners worry about, but piston helps there as well. Tunable block is great too, if you're going to switch DODIC (load) based on mission. If your grendel needs to meet combat specs, consider a piston - you won't lose any accuracy, but you'll gain a few more things.

[jkingrph]

I have two big AR Grendels. One with an Alexander Arms Overwatch upper, the other a home built using an identical 24" Shaw barrel, along with an Adams Arms Piston and bolt carrier. My homebuilt has a Daniel Defense free floated rail type handguard vs the tubular free floated guard on the Overwatch. My range only goes out to 250 yards and I honestly do not see any discernable difference in accuracy. The piston unit is a little heavier, but considering they are both big bench or prone type guns it is insignificant. The piston unit is so much cleaner that it's hard to believe it is an AR. I have three other guns with piston units, two are mid length with AA pistons, on in 5.56 the other 6.5 Grendel, all identical except for barrel and bolt, the other a 5.56 with an Osprey piston( so I could use a standard AR front sight) All have performed flawlessly and are a snap to clean. Again I cannot see any accuracy differences among these guns compared to di types at the ranges I shoot, and really wonder about either system affecting accuracy. With the gas port so close to the muzzle I wonder if the bullet has not already left the barrel before the gas can overcome the piston and bolt/bolt carrier inertia or the gas can travel down the tube and do the same in the di system.

I have not researched history on the two systems, but as far as I know in the US weapons the piston was in much earlier widespread use, the M1, and later M 14. Maybe the di was a solution to simplify and reduce cost and number of parts required , and possibly some weight. Someone else can probably give better answers to this than I.

[The Profit Joseph Sith]

One other point I don't think

[Steamburn]

I have both. One Grendel with a Syrac piston, a Grendel and a 5.56 with Superlative Arms Adjustable Gas Block DI and another 5.56 with an Adams Arms piston.
For the level of shooting I do there is no difference, except, I spent a lot of money on the piston systems I could have used for something else.
Reading through an immense quantity of "expert" opinions in the forums, the matter of having to clean the BCG and environs on a DI system, really doesn't play until you have done about 5000 shots. I shoot about 200 a month, so it will take me a long time before I really have to clean my DI rifles. The trick is to always keep the BCG and environs wet with oil. And again through the "experts", the best oil at the end is plain 10-50 synthetic motor oil.
In any case, I still clean my weapons periodically, but I don't feel the urge or obligation to do it immediately after each session. I sincerely thank all the chemists that made non-corrosive primers and powders a reality.
Note: Soon I will be putting together another Grendel to be used exclusively with cheap steel cased ammo. That might have a different need for cleaning.

[ah1whiskey]

having shot piston rifles for decades and ar-15s and m-16s for as long a time.

i think the DI system is only a hair more dirty than a piston. the DI system saves weight and has fewer moving parts. with proper tuning of the gas-port and buffer weight/carrier weight it can be made to be smooth as silk.

if i was gonna go piston i think i would get the 6.5 AK--