Thread: suppressor pressure

Ok for those who know more than me and there's quite a few of you , can you explain this? I have been developing loads for my rifle using it's muzzle brake. Started at 62gns working up in .5's to 65. had no pressure signs and good speed so stopped as I was happy enough. I settled on 64gns for no other reason than up till then every .5 gave 30fps and after that it was 20 then 10 so figured a good place to settle.
Put a new suppressor on , hardy gen 6 and first shots stamped the brass and very tight bolt. Reduced loads to 62gns and same thing. While still at 62gns I remove the suppressor and didn't put the break on as I didn't have it with me and all things back to how they should be! Not a hint of pressure or stamping or sticky bolt. The hole is the right size in the suppressor and can't see anything wrong with it.. It's done up tight and only just resting on the barrel. I checked the specs and it's the right size for a 7saum . What's the go? Where has all this extra pressure come from?

Pressure over time?
I dont know but the suppressor basicly holds some of the muzzel preasure and releases it slower so maybe this extra time has an effect? or maybe your barrel was hot as with the suppressor?
Im just guessing stuff.

The pressure is retained for longer with the supressor think of it as a bleed off valve.
To work effectively it has to reduce pressure by controlling expansion, reduce gas volume by cooling, and reduce gas velocity by creating turbulence as well as all the above.
Gas operated rifles show the effect of supressors quite well, ar type rifles I believe can be over gassed by them.
Over barrel supressors with larger first expansion chambers than muzzle forwards may let the pressure drop more quickly but im no expert.
Thats my 2 cents anyway.

If you think about it, you are essentially adding barrel length without the significant velocity gains.


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Pressure signs in cases develop in response to the peak pressure. This typically occurs in time at about one-third of the barrel time - the time between ignition and exit at the muzzle - corresponding to when the bullet is perhaps 10% of the way down the barrel. Whatever is causing this would surely need to be influencing pressures then.

I've had two other rifles with suppressors and never had this. Wouldn't everybody have to reduce loads then when using a suppressor? I've heard of velocity gain by using one but it seems no one really sees it or only very minimal if at all. I've gained more pressure than 3gns of powder probably closer to 5 that's almost 10% reduction needed. Seems way to much. It's a carbon fibre barrel so probably has a bigger internal sleeve so being the same od and length I guess internal volume might be a bit less to handle the gases and maybe it ism't enough? but then a 7rm mag would have 5+ grains more powder and that would balance it out as thats more gas to sort on a more standard one. The rifle does seem to be shooting fast. book says a 175projectile 66gns of powder is about 2850fps. I get that with 62.5 and pushing 2950 at 65gns. this was with a magnospeed. The chamber seems long as a no go gauge works easy but I size the brass to suit. 30" jump so no jamming.

My guess has been that the marks are because the air in the barrel and suppressor in front of the bullet can’t escape as quickly as it can when using no suppression or a muzzle break.

Are you running very minimal headspace?

I get the slightest hint of a stamp when using my suppressor as well and this doesn’t show up at all without it fitted or if I’m using older fouling rounds that have more headspace

Possibly I’m loaded up (unsuppresed) very close to the point of a slight stamp or the minimal headspace causes the case head to contact the bolt plunger firmer?

I'm about 2" head space. I've checked all measurements a few times. Only way you could get velocity gain imo is if the suppressor isn't working properly. The point of a suppressor is the same as a car muffler. To slow and remove pressure from hot high pressure air meeting cold. You need pressure to get velocity and it should be lowered as soon as it gets into the suppressor, so no gain. I'm waiting to hear back from hardys to see what they say. The barrel crown is clean so are the threads so doesn't look like escaping gases. But they should come out around the barrel not back down it I would have thought?

You mean 2thou im guessing

The suppressor traps the gas and generates a much longer time/pressure curve. This shows up in gas-operated semi-autos and causes extraction problems in many of them. There are two solutions. One is to open up the suppressor bore which will lower the pressure and the effectiveness of the suppressor. The other is to install an adjustable gas system that can be tuned to suit. Note that .308's etc produce a much greater gas volume than .223's and the problem is worse in the higher case capacity cartridges.

Originally Posted by gundoc

The suppressor traps the gas and generates a much longer time/pressure curve. This shows up in gas-operated semi-autos and causes extraction problems in many of them. There are two solutions. One is to open up the suppressor bore which will lower the pressure and the effectiveness of the suppressor. The other is to install an adjustable gas system that can be tuned to suit. Note that .308's etc produce a much greater gas volume than .223's and the problem is worse in the higher case capacity cartridges.

Yes it will increase the pressure in the gas system but after the bullet has passed the gas block. As in @Puffins post, to give the symptoms described it would need to increase peak chamber pressure ( where the bullet will be only 3-4 inches down the barrel) to give the symptoms described and I dont see anyway that can happen by simply adding a suppressor. I would looking else where. What projectile are you using, what velocity and what chronograph?

Iv also see no significant gain in velocity with suppressors using reliable chronographs and significant sample size.

Originally Posted by sneeze

Yes it will increase the pressure in the gas system but after the bullet has passed the gas block. As in @Puffins post, to give the symptoms described it would need to increase peak chamber pressure ( where the bullet will be only 3-4 inches down the barrel) to give the symptoms described and I dont see anyway that can happen by simply adding a suppressor. I would looking else where. What projectile are you using, what velocity and what chronograph?

Iv also see no significant gain in velocity with suppressors using reliable chronographs and significant sample size.

I would agree that there is no significant gain in velocity but tests here in NZ have shown that good suppressors have increased velocity by a few fps in .22 rimfire rifles.

Yes, gas pressure is increased after the bullet passes the gas port, but pressure is then equal throughout the barrel. The increased pressure after the bullet passes the gas port means that the cartridge is still gripping the chamber wall when the action starts to open. In extreme cases the extractor will rip through the rim of the case. I have cured many of these problems over the years by making an adjustable gas port that can be tuned to suit each rifle, and in many cases it has also been necessary to bore out the suppressor by up to 1 millimeter to give a faster pressure drop. The bigger the cartridge, the worse the problem. Smaller rounds like the .223 have much less gas volume and lose the pressure quicker, although the higher residual pressure in the bore manifests itself by coating the cases in powder residue. An adjustable gas port addresses this situation fairly easily. The sticking case problem is more common with the .308 or bigger cartridges. I have been sorting these problems for other gunsmiths (who don't want to know about them) for many years, and still get to do them even though I am retired.

Originally Posted by gundoc

I would agree that there is no significant gain in velocity but tests here in NZ have shown that good suppressors have increased velocity by a few fps in .22 rimfire rifles.

Yes, gas pressure is increased after the bullet passes the gas port, but pressure is then equal throughout the barrel. The increased pressure after the bullet passes the gas port means that the cartridge is still gripping the chamber wall when the action starts to open. In extreme cases the extractor will rip through the rim of the case. I have cured many of these problems over the years by making an adjustable gas port that can be tuned to suit each rifle, and in many cases it has also been necessary to bore out the suppressor by up to 1 millimeter to give a faster pressure drop. The bigger the cartridge, the worse the problem. Smaller rounds like the .223 have much less gas volume and lose the pressure quicker, although the higher residual pressure in the bore manifests itself by coating the cases in powder residue. An adjustable gas port addresses this situation fairly easily. The sticking case problem is more common with the .308 or bigger cartridges. I have been sorting these problems for other gunsmiths (who don't want to know about them) for many years, and still get to do them even though I am retired.

Yes I agree if we were talking about cycling issues in a gas gun but that's not the situation.. In his 7mm saum the sticky Bolt lift is long after the bullet and any pressure have gone and wouldnt the stamping on the case head is most likely to have occurred at the top of the pressure curve indicating an overload?
Id be interested to see the rimfire tests showing velocity increases with a supressor, are they on line somewhere?

Originally Posted by sneeze

Id be interested to see the rimfire tests showing velocity increases with a supressor, are they on line somewhere?

Chaz Forsyth did the research on this some years ago and it appeared in an article somwhere (possibly NZ Guns?).

I hear what saying gundoc. I talked to hardy's and they want it back. They asked about hole diameter but it measures ok as they thought there might have been the wrong one sent. I did a quick estimate accounting for my bigger barrel needing a bigger sleave to account for the larger OD and if my maths correct that's a loss of almost 25% of the useable space in the rear part of the suppressor. Wondering if that could have something to do with it?