Thread: suppressor pressure

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.

Same powder ,primers and cases. Checked head space and case lengths. 175eld-x 64gns 2225. 2920fps with a magnospeed. 30thou jump. Dropped to 62gns for 2820fps. Same thing. Doesn't do it with muzzle break or straight barrel. A fired case is 20thou longer than un-fired factory brass. It seems to like 3thou or more shoulder bump or will do the same. Sent suppressor back and waiting for info back. Book max is 66gns for a 175 projectile but the eldx have a long bearing surface and stiff jacket so I realize they can generate the same pressure to get the same velocity with a few grains less powder. I reach the book max velocity at 62.5gns. Mind you the book may have a slower barrel or mine could be faster? As long as I make sure the shoulder is back 3thou I get no pressure signs at 64gns. Will up date when I hear back about the suppressor

For the sake of furthering discussion, what barrel length and how many rounds in total showed signs?
Just thinking about this logically, the only way a suppressor could increase peak chamber pressure would be to somehow restrict the movement of the bullet down the bore and the only tool it has is the air between the bullet and the suppressor. By the time the bullet has moved far enough to compress that air given there is a hole and a large capacity in the suppressor itself then the chamber pressure will have dropped considerably. We can completely block the muzzle safely with insulation tape for the same reason, ie it has no negative effect because by the time the bullet is 1/2 way down the barrel the chamber pressure has dropped by half. So On the face of it i would be treating the suppressor as a red herring.
Dropping 2 grains and still seeing the same issue is interesting, my first thought would be maybe to much shoulder bump which can give false pressure signs.
When you say fired brass is .020 longer than new is this a head space measurement or over all length? Either way that's a lot, way more than I would like to see and may add to the head space theory. The fired primer/ cratering extruding a little can give a false reading here so make sure its seated back in to the primer pocket before checking the measurement.
I havnt used the 175eldx but the 6.5mm 143gr defiantly required more pressure to reach the same velocity as the Berger equivalent. Iv also found 2225 can sometimes be a a bit peaky at the hot end and Iv got a hunch its more so in cold rather than hot temps.

I'm thinking headspace issue.
Nothing else makes sense.

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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?

After re-reading my post I should amend 'increased pressure' to 'maintaining the high pressure for a longer time period' (ie; my earlier reference to the time/pressure curve).

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?).

Originally Posted by gundoc

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

Thanks , ill see if I can find it.
Iv see a few claims from he past disproved with updated tech, Im wondering if the accuracy of today's magentospeed/labradar chronys will show similar increases. Can you remember what suppressors he was testing?

24 inch proof barrel. I haven't used new brass yet. Shot count was a bit over two hundred by the previous owner so about 300 now. My loads are slightly less than his as I changed from 2217 to 2225 and reduced the charge. I did try 2217 and had stamping so went to the slightly slower powder. I pulled a couple of his loads to check against my scales before starting. Neck tension seems tight as bullets are very hard to pull. Measuring sized cases looks like 3-4 thou bullet grip. We have had a no go gauge in the chamber and the bolt closed so that explains the difference between new and fired cases. I was measuring the head space not overall length. I trim to 5 thou less than max, perhaps I should go the full 10thou?
As an up date I fired the rifle yesterday with the left over 62gn loads and it did the same. These were fine previously. I fired some 64gn loads and they were fine. The loads are compressed but I wouldn't have thought not overly. But I am wondering if the cases after being loaded are pushing the shoulder forward slightly or changing in some other way? Spinging back after sizing? When I get a chance I'll do some more measuring before ' during and after loading but this will be a couple of weeks away. I check weigh each load on another set of scales and measure each to ogive. My jump is about the same as the previous owners specs he supplied and documented. Would like to anneal but need to make a new bowl to fit the short cases first

Originally Posted by sneeze

Thanks , ill see if I can find it.
Iv see a few claims from he past disproved with updated tech, Im wondering if the accuracy of today's magentospeed/labradar chronys will show similar increases. Can you remember what suppressors he was testing?

His testing involved all of the commercially available suppressors (Parker-Hale, Unique, WCG, Manders, etc) at the time. I think he used a Brno bolt-action rifle for testing.

Originally Posted by r87mm

24 inch proof barrel. I haven't used new brass yet. Shot count was a bit over two hundred by the previous owner so about 300 now. My loads are slightly less than his as I changed from 2217 to 2225 and reduced the charge. I did try 2217 and had stamping so went to the slightly slower powder. I pulled a couple of his loads to check against my scales before starting. Neck tension seems tight as bullets are very hard to pull. Measuring sized cases looks like 3-4 thou bullet grip. We have had a no go gauge in the chamber and the bolt closed so that explains the difference between new and fired cases. I was measuring the head space not overall length. I trim to 5 thou less than max, perhaps I should go the full 10thou?
As an up date I fired the rifle yesterday with the left over 62gn loads and it did the same. These were fine previously. I fired some 64gn loads and they were fine. The loads are compressed but I wouldn't have thought not overly. But I am wondering if the cases after being loaded are pushing the shoulder forward slightly or changing in some other way? Spinging back after sizing? When I get a chance I'll do some more measuring before ' during and after loading but this will be a couple of weeks away. I check weigh each load on another set of scales and measure each to ogive. My jump is about the same as the previous owners specs he supplied and documented. Would like to anneal but need to make a new bowl to fit the short cases first

Ok so your getting spasmodic stamping over a 2gr range with No suppressor? What is the action?The head space issue may need checking , you can add shims to the back of the gauge to see just how far out of spec it is though the .020 from new brass is an indication. If its not bad and just a poor cambering job ,while the best option is to set the barrel back a thread and correct it, you can compensate by fire forming brass correctly and setting your sizing die properly ,bumping the shoulder no more than 2-3 thou. To much shoulder bump will give the signs you are seeing and will lead to case head separation. Compressed loads are fine as long as the neck tension holds the bullet. If the compression is to great the the bullet could be creeping out over time and lodging in the lands which can cause a pressure spike. Shoulders springing back should show as "tight to chamber" and generally wont add to pressure.
Your chamber is too long so trimming more wont help.
So it could be a number of things and slow work out through conversation like this, if you want drop me a PM, you can bring all the bits up and see if we can/t work through it.

Thanks Sneeze. I went back over everything while I was waiting for Hardy. He couldn't find any fault with the suppressor but sent me a new one anyway. Sorry it's taken so long to add to this thread but have only just had a chance to shoot the rifle. I rechecked my die setting and are around 2 thou shoulder bump. Plenty of neck tension, 3thou and very hard to pull so no movement there and verified with measuring. When I fired it with the 64gn load there was still pressure signs but not as bad. I went back to my trial loads and looked closer. The book stated 175gn bullet with 66gns should be about 2850fps. I was using different cases and a 175gn eldx which are known to have a long bearing surface and tougher jacket than the eldm. I was getting 2815 at 62gns and 2847 at 62.5 upto 2920 at 64. Since velocity is a result of pressure I decided I was at max with the brake and the suppressor may have had a slight back pressure and pushing it over the edge and causing problems. So i reduced back to 61.5 and went up to 62.5 which seemed ok and was still more speed than I had expected. It doesn't group as good yet so I intend to load some up and fire through both brake and then suppressor. The previous owner used 2217 at 64gns I think with out checking but said it was getting up there ( I did pull a couple and our scales seemed to be the same) If the accuracy doesn't improve I'll load some with that. Have manage to anneal the cases now so interesting to see what difference that makes. Wounder it the slightly faster powder will have a different affect on launching the bullet a bit quicker from the start? Also want to shoot prone so the bi pod spikes can dig in better than from the bench. Interestingly the bipod seems to come lose after a few shoots. I tighten it enough that I'm worried the front stud will pull out but still have to re tighten. So there's my conclusion , Being right on the max and just a small thing to make it to much. My 7rm ran 70.5gns of 2225 for 2850fp with the 180eldm. I was hoping the 7saum would do 2750 and be happy with anything more. I am surprised how strong the cases seem compared to others so maybe it doesn't show pressure signs till quite above where you would expect? Least I've got more excuses to do some more shooting