According to Noveske a piston doesn't matter on a suppressed weapon.
http://www.defensereview.com/novesk...ecce-carbine-john-noveske-interview-part-one/
Crane: You’re not doin’ a piston gun, right?
Noveske: No.
Crane: Do you have any plans to do a piston gun?
Noveske: We have piston plans, but we don’t have any plans of putting it in production, because it’s…I don’t think it’s necessary. I’ve got piston guns here from other makers, and they’re dirty, and I don’t see…
Crane: Whadya’ mean "dirty"?
Noveske: Open up the bolt and look inside, and it’s dirty inside. The whole thing about them running clean is not necessarily…o.k., let me back up. I only run the guns with suppressors for testing when I did my comparison, and with suppressors, direct-impingement and piston-operated were both very dirty, ’cause the blowback comes to the chamber, not the gas tube. And, I’m not real happy with the piston systems that I’ve shot and examined, so it’s just to me, it’s not…
Crane: Well, the piston…the advantage for a piston with a suppressor on there is supposedly it doesn’t blow all the gunk back in your face.
Noveske: O.k., but what you’re not paying attention to is that all that crap comes back through the chamber, not the gas tube. On a piston gun or gas-impingement, the case is being extracted while the suppressor is still under pressure. Now you have all the pressure in that suppressor exiting both out the front and the back.
Crane: Right, but you’re saying the piston gun doesn’t solve that?
Noveske: It does not solve that. They’re both dirty.
Crane: So then how come you hear about guys saying yeah, when they’re shootin’ the direct gas impingement guns suppressed, or whatever, they’re gettin’ a lot of gas and particulate matter in their face, whereas with the piston, that it dissipates that a bit, or whatever.
Noveske: Maybe they had a different experience.
Crane: Hm. So, in other words, you’re saying that basically the piston doesn’t really offer any real advantage for that.
Noveske: What I’m saying, with a suppressor, direct-impingement and gas-piston both run dirty, and even a blowback gun or a delayed-blowback gun, like an H&K [Heckler & Koch], or any other operating system–I don’t really care what operating system you have–on an auto-loader, with a sound suppressor, they’re gonna’ all run dirty.
Crane: Right. Now, is a piston gun gonna’ put any less gas and particulate matter in your face, or are you gonna’ get the same amount?
Noveske: All a piston gun is gonna’ do different from gas impingement with a suppressor is reduce the amount that is coming through the gas tube. The piston gun is gonna’ eliminate that. I am not a scientist, but from my observations in shooting and examining the guns afterwards, it appears that the vast majority of the gas coming through is coming through the chamber. And, one example is go look at any of the HK91 or HK93-type rifles. Those have the fluted chamber and delayed blowback, and the cases are always black just like the case fired out of the gun with a suppressor. That’s because the case is extracting while it is still under pressure, and you have gas blowing back along the case as it’s blowing out, and covering it with carbon. And, that’s what’s happening with any autoloader with a suppressor. The cases all have carbon on them, because gas is escaping around the case out the chamber and into the receiver.
[DefenseReview received the following post-interview via email from John Noveske: "Also, we should mention the poor choice of platform for the piston conversion on a round receiver bore as found on the M16/M4 system. All other piston type systems out there utilize a railed receiver design, like the M14, AK-47, M249, FAL and so on. The round receiver bore design used on the M4 is only acceptable for the standard op system. The carrier and bolt expand on axis with the bore under the normal gas impingement cycle, but on a piston gun, you run into off center impulse issues with carrier tilt and incorrectly designed carrier contact points. Some designs attempt to address the carrier tilt problem with over sized carrier tails and rollers. I do not believe the receiver extension should be used in this manor. I know many people are very happy with their piston weapons. This is not meant as a knock on the piston conversion systems out there, but as a philosophical dialogue focused the new physiological relationships applied to the M16/M4 platform through the introduction of an operating system which has traditionally been applied to receivers with rails for the bolt and/or carrier. I would rather see an entirely new weapon system designed for the piston from the ground up. I believe there several outfits currently working on this."]
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I think with a good design it is much more reliable. 
