I am firmly in the DI camp, and an experience last night reinforced my stance even more. Two guys I was shooting with had very comparable rifles. Both 7.5" barrels; both using the same SiCo Saker. One was a Sig 516 (piston); the other a Springfield Saint (DI).

I am unsure of the ammo being used, and I concede this uncertainty takes some wind out of the flash assessment sails; I'll follow-up with them, to see if I can find out. I am also unsure whether the Sig's flash was being emitted at the muzzle or the piston; piston flash is a real thing, and I suspect even worse on a system that short.

The Sig was much, much louder than the Springfield, and had much, much more flash.

A lot of piston guns have adjustable gas, which is a boon to a silencer; they also don't "shit where they eat", as DI detractors like to say, but they can make significant sacrifices in suppressed performance at the bottom line, which is noise and flash.

Thanks for another informative post. I wasn’t familiar with the term “piston flash,” so I researched how piston guns work and I assume it’s the venting of gas between the gas block and the forward end of the piston as shown starting at 0:40 in this video:

https://www.youtube.com/watch?v=GvUjVYg8JBk

Although it would probably never be important to me in any significant way, it’s just one more reason why I’m glad that I never got sucked into the “piston guns are better” fantasy, and one more thing to know if I’m ever asked for my opinion about the system.

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I wasn’t familiar with the term “piston flash,”

I don't know that it's a term in common usage; I just decided on that word combination, to describe the phenomenon. I suppose the potential for it to be a problem depends largely on the design aspects of the gas system, and the the length of said system and/or the gas port and vent's distance from the chamber.

As described in the HK416 video recently shared by Para, the G36 and 416 system is especially prone to a flash, due to how it vents extra gas. I suspect what I observed in my friend's rifle, if indeed piston flash, was due largely to the short length of the system.

Well, the term makes sense, and if it didn’t already have a recognized term to describe it, I’m tempted to believe it’s another one of those “dirty little secret” things that fans of certain equipment and systems would prefer not be mentioned in public.

For anyone who may be as ignorant of the piston venting as I was, here is an excellent slow motion video of an actual, not animated effect.
https://www.youtube.com/watch?v=3KYScJo4_s0

And for piston fans, does it matter?
As I say, not to me, but it is something to be aware of.

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Originally posted by KSGM:

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I wasn’t familiar with the term “piston flash,”

I don't know that it's a term in common usage; I just decided on that word combination, to describe the phenomenon. I suppose the potential for it to be a problem depends largely on the design aspects of the gas system, and the the length of said system and/or the gas port and vent's distance from the chamber.

As described in the HK416 video recently shared by Para, the G36 and 416 system is especially prone to a flash, due to how it vents extra gas. I suspect what I observed in my friend's rifle, if indeed piston flash, was due largely to the short length of the system.

Neither the G36 or HK416 were designed for suppressors or have adjustable gas blocks (except for some later 416 models with AGRs). They control pressure by venting out the front of the gas block. I believe the “superiority” of DI in this specific case is mostly an accident of design, because this wasn’t something that was considered way back when the AR-15 gas system was designed.

Remember that when many guns were designed, suppressor use wasn’t a consideration in most cases. Suppressors were rarely used outside of certain special forces, most suppressors were designed for noise reduction first and almost no one cared much about flash elimination.

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the “superiority” of DI in this specific case is mostly an accident of design

Absolutely. But it is interesting that something accidental is still superior to things built after the fact; things with more of a focus on better performance in the area that the accidental thing still reigns superior.

I imagine flash suppression has always been a priority for military applications. If a silencer being pitched to the military had incredible sound reduction, but more flash than the unsuppressed host, I think the silencer manufacturer would be sent back to the drawing board.

These are guesses though.

I've been wondering about this also. I found out the hard way with the FS2000 that the gas piston shoots a jet of flame out the side in some designs. Flame-cut my sling, and got my hand once, too. Of my 5.56 rifles, only my Bren 2 is a gas piston design, and it's going to be at the bottom of the list for tuning once I get my cans home. I had been thinking I'd have HBI do my OEM piston, but then I saw that KNS came out with their own about a month ago:

https://knsprecisioninc.com/kns-czech-valve/

42 gas settings, and according to that review, there's no gas port flash. Going to see if I can ask a few of the guys on Reddit who bought some. If they don't know, I guess I'll find out eventually.

I do think that for most of my purposes with a rifle, I'll stick to DI. I know it makes a God-awful mess, but that's what ultrasonic cleaners are for, right? Right?

So without having access to either of those guns to drag them out and disassemble, I'm guessing naked short stroke tappet for the gas system? If that is the case, would you think there may be a difference in a sig 550 like long stroke inside of a sleeve that may change the release of the gas? The sleeve being the potential difference maker.

Disclaimer: I'm on my first cup of coffee so none of the above may be true or smart.

Interesting discussion.

Do you think caliber and barrel length affect this? Since .300BO is supposed to complete its burn within 9" or so, might those be factors?

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long stroke inside of a sleeve that may change the release of the gas?

It does seem likely that a long stroke wouldn't be as susceptible.

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Do you think caliber and barrel length affect this?

Probably. It stands to reason that a longer barrel with longer gas system, or even a longer system on the same length barrel as is being criticized, would have less of a blast from the gas system vent.

On top of that, most every piston rifle I have shot has a sharper recoil impulse. DI always seems softer.

Not my experience. A SCAR17 is way nicer than any AR10 I've owned.

How does the SCAR perform strictly in the noise department, when compared to a DI?

Honestly I don't have the tools to tell you the answer on that. And resources I normally use like Pew don't have that kind of data. I love the SCAR suppressed (as is surely clear around here from my posts for interchangeable gas jets, easy to change barrels and aftermarket enhanced parts etc.) But I can't tell you how much better or worse it is on just noise than other things of a similar nature. Its definitely more pleasant to shoot a tuned 17 than the AR10's I own using a surefire, but I haven't spent any effort making the ar10's pleasant because in the end I am never going to rely on one. All of the .308's are loud to me...
As a 16 and 5.56 I can't actually report anything noticeable between a MK18 or 11" colt and a CQB scar with an RC2.

Touching on the subject of adjustable gas. I have adjustable gas blocks on most of my ARs, all of which are DI.

They can be tuned solely for suppressed use. And in all of my rifles, can also be tuned to operate properly with and without the silencer attached.

I get much less gas to the face, and a softer recoil impulse, when utilizing an adjustable gas block to tune the rifle. Most rifles are overgassed so that manufacturers don't get complaints from people who use ammo loaded to lower pressure, like some of the Wolf steel cased stuff.

Shoehorning a piston into a rifle that wasn't designed for it, never seemed like a good idea to me. The DI system that uses the tail of the bolt as a piston is an elegant bit of engineering.

The amount of fouling that enters into an AR with or without a suppressor is not enough to stop a quality weapon through more rounds than anyone is likely to be carrying at any one moment.

Wipe out the fouling after 3-500 rounds and add some quality lube. The rifle will keep running. White gloves need not apply, a silly waste of time. I don't even use cleaning solvent on anything except the bore.

Last weekend I shot two back to back high round count three gun matches over a period of 3 days without cleaning my AR. Rough guess about 600 rounds in total.

Not directly related to the OP but IndianaBoy post makes me ask this. I have no problem making my MK18 and 6933 happy suppressed with some modest tuning. But its not adjustable tuning as I simply don't care if they run or not unsuppressed. But I have always wondered if there is someone who makes an adjustable gas system for an AR15 that is somewhat bulletproof. I see them with set screws (complete yuck), clamp on (double yuck), single cross pin(approaching ok but not really) and several other worse ideas, etc. for attachment. Is there someone who makes a barrel that takes dual taper pins and provides an adjustable gas system attached like all good gas blocks should be attached?