Backpressure and suppressor design

ddavis

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So I finally received the K&M M17s bullpup I posted a question about a while back. It shoots great un-suppressed, but that's not why I bought it.

Some background which leads to my question:
The rifle has a 7 position gas block. Position 1 has a 0.025" hole, per the owner/designer. That's the only one I know the size of and it seems really small. It is recommended to be on position 2-3 to get the rifle to run with a can. Position 5 is the factory setting and runs great unsuppressed, which is the case with the M80 ball type ammo I tried. My gun locks the bolt back violently with a single round loaded and double feeds within 3-4 rounds at position 1 with both my TBAC Ultra 7 and an AAC Cyclone. I sent it back and it was tested with the owner's can (either a YHM Phantom or SDN6) with no issues and locked the bolt back on position 2 like he expected. I'm sure I could load some weak ass ammo and make it work, but I really want to run factory ammo for this gun.

What is so different about either my TBAC7 or AAC Cyclone that causes so much backpressure? I also have a TTF Archangel (which I think is the same as the current Crux cans/renamed) that I'll be testing when I get the gun back also. Looking at all three .30 cal cans I've got, the TBAC has the largest blast chamber and is shortest by 2" which I would think would help reduce backpressure. The bore seems to be the same on all of them.

I know OSS cans are known to have very little backpressure due to design, but what else influences backpressure? Would a SiCo Hybrid with its .46 bore help? Running a flash-hider vs a brake mount? FWIW I was running a flash-hider for the TBAC.

Thanks,
-Dan
 

ddavis

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The Deadair Sandman was designed for lower back pressure as well.
This is the kind of info I was looking for. If I can't get any of my other cans to work, I may be in the market. But I really don't want OSS and I'm not even sure why. I'm also interested in why/how it has lower back pressure, just because I'm curious.

-Dan
 

chrismartin

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Different baffle designs will slow the exiting gasses from coming out the front. This can also cause the gasses to go back into the action. Different angles on the cones, scallops on the surfaces, pass-through holes on the baffles, etc. All change where the gasses can exit. Even looking at the baffles won't tell you the entire story though. The SWR/Silencerco Octane pistol silencer is known for higher blow back than, lets say the AAC TiRant. My AAC Evo is worse than the TiRant, but the baffles are both K baffles. I can't "see" the difference, but minute changes in the design can change the flow of the gasses.
Also, I have noticed that the industry has changed over the years. The "Mil-Spec" measuring location (1 meter left of muzzle and 1.6 meters off ground) has lead to a lot of companies tuning silencers to reduce the sound at that location to play the marketing games. Others (like dead air) have noticed that if they reduce back pressures, it not only reduces problems with semi-autos, but it also reduces the sound at the shooters location (Generally anyway). However, capturing this data and marketing it to users can be difficult, so you don't find it much. Silencer Shop and folks with multiple meters (Like the folks at NFATAlk.org) have started capturing data from mil-std location as well as left and right "ear" locations.
However, you just have to find silencers known for reduced pressures, and it's just marketing type info that lets you know that.

Allen Engineering AEM silencers (Over the barrel design) are also lower back pressure, however, they have barrel design restrictions due to the mounting system. Griffin also has two reflex silencers that do the same thing (one .30 cal and on 5.56)
 

Cheyenne Bodie

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Chrismartin explained the nuts and bolts of it very well. Basically, it comes down to how quickly the gasses can be released and/or how much internal volume they can find to expand into. The baffled rifle suppressor is essentially a pneumatic Tessla valve, where the outward flow of gas is folded onto itself and the bullet is "skinned" of those gasses on its way through the bore— but as the flow is due to expansion there will be flow back all the same.

HSS supps have very low backpressure, but they're kind of unremarkable at silencing. Like Zak said, that's the norm, but not the rule. Deadair has low backpressure and are very reliably built, but they are unremarkable at silencing. Rugged seems to split a good difference between back pressure and noise reduction, but that's mostly just my own observations.
 
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Evolution 9

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The Surefire socom silencers have very low back pressure (shallow dome shaped baffles as opposed to aggressive cones in the TBAC). For bolt guns I run TBAC, for semi auto I run Surefire.
 
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mikesmith13807

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This is the kind of info I was looking for. If I can't get any of my other cans to work, I may be in the market. But I really don't want OSS and I'm not even sure why. I'm also interested in why/how it has lower back pressure, just because I'm curious.

-Dan
I only know what I've read on the internet, but I have read a lot because I'm very interested in this question. My understanding is that back pressure in conventional designs is primarily improved by internal volume, and maybe also by things like aperature diameter and internal design. That's why reflex designs seem to do better, check out Amtac if you aren't aware of them, they are the primary U.S. manufacturer focusing on that approach that I know of. Of course AEM does well, but they are in a special category due to the mount IMO.

In general the designs that generate good meter levels at the muzzle are much louder at the ear, usually significantly over 140 DB. This would probably be worse with a bullpup since the ejection port is closer. That's considered the most direct measurement of back pressure that is easily accessible. Increase in bolt speed would be another but I haven't seen anybody doing those measurements yet other than manufacturers for marketing purposes.

OSS took a different approach, instead of trapping it like a baffle design they are trying to redirect the gas and let it flow around inside and eventually out the front, hence the term "flow through". The idea is to give it some time to slow down expansion before hitting the atmosphere. It certainly generates higher muzzle meter levels but seems to do quite well at avoiding increases in back pressure and keeping the levels at the ear lower. Based on the meter data I have seen they tend to do better at the ear without gas adjustment than most competitors. Your negative impression of them is probably related to the fact that previous designs were pretty heavy and came out at a time when many people were not testing levels at the ear, so the dominant opinion on the Internet is that they are loud and heavy.

Other companies are now copying their approach to some extent--MaxFlo 3D, LaRue, Delta P, and Thermal Defense (a newcomer using the 3D printing approach) are some that come to mind.

A lot can certainly be improved with conventional designs once you start adjusting the gas on an AR, but you may not have enough adjustment available to you and may want to try some other designs if you are considering another purchase.
 

ddavis

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I've learned more from this thread than I thought I would have, thanks guys for all the knowledge. The rifle comes back tomorrow so I'm going to give it a try with the only .30 cal can I own that I haven't tried yet. If it doesn't work with the TTF/Crux Archangel then I'll probably have to buy a can that it will work with. I don't need it to be super quiet, I just need it to not blow my eardrums out when I'm night hunting for pigs, and I'd also like short and light. I doubt I'd ever go back to something steel. The TBAC is about the perfect .30 cal can IMO, it's too bad this rifle can't adjust the gas down further.

-Dan
 

Almonz

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Everyone here has explained it pretty well. In a nutshell basically the more gases the can traps, the more backpressure you'll have generally speaking. Without getting into baffle design and engineering that is what is happening inside of the suppressor. Me being a lefty and shooting a suppressed AR backpressure is a huge issue for me sometimes.
 

Potss

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On 5.56 the OSS is a great option for piston guns, in fact I'd say it is the best option for them in most cases.

But on .308/7.62x51 it is too loud at the muzzle on 16in barrels, 147-150db. The washback from that negates the at the ear numbers as Dead Air and other manufacturers have confirmed. Or to state it another way, the muzzle is so loud that it will be louder to the shooter than the chamber unlock. So while the OSS would be a solution to the back pressure issues you are having, it would be pretty damn far from quiet, especially for such a big, heavy, and expensive can.

The better solution would be one of the other cans recommended ITT like the Dead Air Nomad. But a better solution still for someone who isn't chasing dbs like you say would be on the .30cal short cans like:

1.) Energetic Armament Vox K. 4.4in DT or QD (QD is anything Omega like Keymo or Area 419). 9.9oz. ~$600. Uses wipes, supposedly FH endcap soon.

2.) Rugged Suppressors Micro30. 5in QD. 11.8oz+ 1.5oz mount=13.3oz. ~$700.

3.) Rugged Suppressors Radiant762: 5.1in (short config) QD. 9.4oz+1.5oz mount=10.9oz $945. And you get a can that can go long config and be even quieter if you want.

4.) Dead Air Sandman-K. 5.4in QD (~4.8in added). 12.8oz+3.7oz mount=16.5oz. ~$569. Can use flash hider end cap. Fantastic but heavy QD system.

All are supposedly (and I say supposedly because no one has independently verified with a proper meter so this is off manufacturer claims, but I trust Dead Air) right around 143-145db at the muzzle. Again, not quiet, but below the threshold where you'll get splashback from the muzzle. In other words, the chamber unlock and the gasses coming out there will still be the loudest thing to the shooter which is what you want. All have very low back pressure due not just to design but also to the small size and limited baffle stack. Keeps your bullpup shorter too, which I assume is the point. I don't have a K&M M17s to test any of these, but if the AR data is anything to go off of one of those four would be your best bet.
 

mikesmith13807

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On 5.56 the OSS is a great option for piston guns, in fact I'd say it is the best option for them in most cases.

But on .308/7.62x51 it is too loud at the muzzle on 16in barrels, 147-150db. The washback from that negates the at the ear numbers as Dead Air and other manufacturers have confirmed. Or to state it another way, the muzzle is so loud that it will be louder to the shooter than the chamber unlock. So while the OSS would be a solution to the back pressure issues you are having, it would be pretty damn far from quiet, especially for such a big, heavy, and expensive can.
I'm not trying to be a (insert favorite insult) here, but you really need to stop spreading bad information. I understand your theory, but when somebody has used an actual meter and shown something different than what your theory predicts, your theory is irrelevant. Pete at TFB measured the 7.62 on a 16" 7.62 rifle and got several DB under 140, which is a lot better than most competitors would probably perform (I say probably because there isn't much ear data available for this caliber).

https://www.thefirearmblog.com/blog/2018/09/01/oss-suppressors/

You also need to stop saying big, heavy, and expensive. All 3 of those are easily shown to be a misrepresentation simply by comparing data available on every manufacturer's web site.
 

Qazwsx

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I have found surefire suppressors, specifically the socom issued 556 cans to be very high back pressure and a pain in the ass to shoot left handed on 10.5 and 14.5” guns
 
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Potss

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I'm not trying to be a (insert favorite insult) here, but you really need to stop spreading bad information. I understand your theory, but when somebody has used an actual meter and shown something different than what your theory predicts, your theory is irrelevant. Pete at TFB measured the 7.62 on a 16" 7.62 rifle and got several DB under 140, which is a lot better than most competitors would probably perform (I say probably because there isn't much ear data available for this caliber).

https://www.thefirearmblog.com/blog/2018/09/01/oss-suppressors/

You also need to stop saying big, heavy, and expensive. All 3 of those are easily shown to be a misrepresentation simply by comparing data available on every manufacturer's web site.

Yeah, as usual it is you spreading misinformation for your pet brand. It is no theory, it is a confirmed fact as Todd at Dead Air has pointed out multiple times, you need a different setup to catch it properly.

And no, even comparing the OSS titanium it is still long, heavy, and expensive compared to the K .30cal cans...which are also going to very probably be quieter to the shooter.
 
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SupressYourself

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Long, heavy, and expensive are all relative. Personally, I'd put up with a few of those to get a can on my AR 10 that doesn't create major back pressure, as long as it's still "hearing safe" at my ear. The OSS appears to do that. Those with the Sandman, can you put it on an AR and not be over gassed?
 

mikesmith13807

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Yeah, as usual it is you spreading misinformation for your pet brand. It is no theory, it is a confirmed fact as Todd at Dead Air has pointed out multiple times, you need a different setup to catch it properly.

And no, even comparing the OSS titanium it is still long, heavy, and expensive compared to the K .30cal cans...which are also going to very probably be quieter to the shooter.
Seriously? Now OSS products have to be comparable to K CANS or they're no good????!!!! Talk about moving the goal posts.... You're just being ridiculous at this point.

And when the data disproves your hypothesis, you just say the methodology is wrong? Please, tell us what the setup should be to discover this critical elusive phenomenon that everybody is missing!

Why can't you consider it's possible that what one manufacturer saw happening with their design might not be happening with another manufacturer's very different design?
 

mikesmith13807

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Long, heavy, and expensive are all relative. Personally, I'd put up with a few of those to get a can on my AR 10 that doesn't create major back pressure, as long as it's still "hearing safe" at my ear. The OSS appears to do that. Those with the Sandman, can you put it on an AR and not be over gassed?
To be fair to Dead Air, they have made it obvious that they have/are trying to make progress with the back pressure issue. I think it was Todd on another forum who said they work hard to balance ear and muzzle levels, and from the data I have seen they do seem to do better than many others in that regard. They and others like Q should get credit for their efforts in this area. I don't think they do as well as the guys who aren't limiting their designs to baffles like OSS and NG2, but it's getting better.

I am always interested in meter measurements at the shooters ear, but an even more direct and easily quantitative measurement would be to put a product on a normal AR without any gas adjustment, fire it on full auto, and measure the change in rate of fire. The test equipment would be relatively cheap, so anybody with access to a full auto lower could start doing this.
 
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SDet

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I believe thunder Beast has a few videos showing the difference between muzzle and ear numbers on a bolt action.
 

ddavis

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I saw they already replied to your thread over there ddavis, but this guy seems to be having no problems on the K&M M17s 308 with an Omega. Seems like you just need a bit less back pressure then, a lot of modern cans will get you there like the Nomad.

Thanks for the heads up. Good to know that it can be made to work with some suppressors. If I hadn't just spent ~$2k on the rifle, I could probably swing getting a new can in a year. Hopefully I can get the paperwork going on a Nomad in the next few months and have a working rifle in 2021 or so. Thanks ATF.