DI or Piston?

[MistWolf]

I question this. The M14 has a 22" barrel with the gas block quite far out. It has a unique gas-piston design that has internal parts meant to compensate for different levels of gas pressure, and there is movement inside the block before the op-rod starts to move. The op-rod is long, heavy, and a bit flexible. And the long barrel means the bullet is already moving very fast before it passes the gas port, likely more than is true in the shorter barrel, shorter gas system setup of the HK. Just because X is true for the M14 doesn't mean it's necessarily true for a different rifle with a differently configured operating system.

I don't have facts on whether the HK bolt starts moving before the bullet exits the muzzle, but it's better to refrain from stating some other source is just wrong when you're basing the argument on an apples vs. lemons comparison.

The piston of the M14 cannot move without moving the op rod. While the gas system of the M14 isn't exactly the same as the HK, the science remains the same. If that's not enough to convince you, study slow motion video of self loading rifles being fired. The actions of gas operated rifles don't start moving until the bullet has left the muzzle

[JC5188]

Anecdotal bit here.

I taught at a large shooting school where each class was 20 to 40 students. Most by far, had shitty off brand cheap DI ARs. There were at least 18 DI guns (mostly shitty ones) for each piston gun. Most were LWRC and HK. Even with such a dramatic ratio of DI to piston, and even with the obvious quality differences, most of the equipment failures were from the handful of piston guns.

It's just an anecdote, but it was a glaring example of "if it ain't broke, don't fix it".

What types of failures? Was there parts breakage? Mag issues? Was it something that was constant, or a FTF here and there? If you had to estimate a MRBF for the piston rifles, what would you guess? How about the DI rifles?


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[BadgerPeak]

This was a while back, so my recollection of details won't be that great. I recall parts breaking, and becoming disassembled as well as some malfunctions tat mimicked badly over gassed DI guns. LWRC was the most common on our ranges, but there were others. Most of the DI guns were DPMS or Bushmaster.

To reiterate, good modern piston guns can be great, and I really liked the 416s, but I don't see them as a solution to anything.

[tylerw02]

It's 2017, does this topic really need rehashed...again?


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[Arctic1]

Arctic,

I'm a very public target at the moment, so I won't get into detail. My experience with these weapons in a work context spans 5 years in addition to over a decade of casual experience, instructing, etc.

I have no doubt your resume beside mine would be comical (in your favor).

As stated in my previous post, before I saw this one, people should avoid looking to you or me as authoritative on the DI vs piston debate just because we have used them on a two way range. Anecdotes have value, if treated as what they are. They are data points. The more the better.

A civilian with lots of ammo and several of each type of weapon has every right to share his valid experience on the topic. Heck, if he's been to Phoenix in the summer and central Idaho in the winter, he has experienced weather conditions harsher than most combat vets.

I've been that civilian, and I've also used them for work. With that experience, I prefer DI. Others prefer piston.

I have no skin in the game. What others choose doesn't affect me. Just offering my perspective.

Respectfully, 42,000 weapons is not anecdotal. Continued use in harsh environments is very different than a civilian going to the range one day with very hot or very cold weather.

I have tried getting some hard data on failures, but the reports are not for public distribution and seeing as I am not AD anymore I don't have easy access to this info. Still working on it.

What I do know is that per the contract, HK warranties all parts for 10,000 rounds, meaning that if a failure occurs before 10,000 rounds they will fix it for free.

Of the actual failures I saw during my 5 years, they were pretty minor, and happened during training; 1 loose castle nut, one piston with a gas ring missing, one bolt override and one CH broken on a frozen gun after my PL tried to pull it to the rear.

If I get some more info I will share.

Our HK417's hold sub-MOA at 800m with the current issue duty round, so accuracy is not an issue with piston guns.

What I don't get with this whole debate is that whenever someone asks for advice on which route to go if they want a piston gun, lot's of folks reply with "Ar's don't need a piston".....as if that is what the guy was asking. And most of the time the people commenting negatively about piston AR's have never even fired one.

[sasquatchoslav]

Spartan I think your mission is complete. Make 2 posts on the most beat a dead horse topic ever and vacate. I seem to see this repeat on the few forums I. "DI or Piston", start the fire and grab the popcorn! Well played sir.

[Spin Drift]

Spartan I think your mission is complete. Make 2 posts on the most beat a dead horse topic ever and vacate. I seem to see this repeat on the few forums I. "DI or Piston", start the fire and grab the popcorn! Well played sir.

vicious_cb tried to warn us…


Arctic1, what is the issued ammo for the 417?

Are all the 417s issued with a 20" barrel?

What is the most issued optic on that rifle?


Thanks for the info.

[sasquatchoslav]

vicious_cb tried to warn us…

I've enjoyed this thread immensely it helped me get through a 2 hour flight delay the other day. Cheap entertainment via airport wifi.

[Arctic1]

vicious_cb tried to warn us…


Arctic1, what is the issued ammo for the 417?

Are all the 417s issued with a 20" barrel?

What is the most issued optic on that rifle?


Thanks for the info.

The issue round NM258, made by NAMMO.

STANAG 2310 compliant 168gr FMJ, with a V0 of 805m/s (2641 f/s). The HK416 manual states a V0 of 850m/s, but I think that was the previous round.

All guns are 20" guns with accurized barrels. They are semi-auto only.
They come with a Schmidt & Bender PMII 3-12 x 50 and B&T Rotex III suppressor.

[Clint]

What I don't get with this whole debate is that whenever someone asks for advice on which route to go if they want a piston gun, lot's of folks reply with "Ar's don't need a piston".....as if that is what the guy was asking. And most of the time the people commenting negatively about piston AR's have never even fired one.

True.

Perhaps the most reasonable advice would be:

If you want a piston AR, get an excellent HK416 based variant, skip any add on retrofit options or just get a quality DI AR.