Thread: Suppressors and pressure

I sorted out a preliminary load for my reasonably new 6.5PRC (sub 100 rounds), 53.5grn of H4831sc which is 1.2 grain under Hornady published max. This load showed the best accuracy with 143 grn ELD-x seated at mag length which was about 3/4" for three shots. ADG brass new, never fired. Plan was to use this load for the first 100 rounds (all of this projectile I've got) and then change to 147 ELDM (because I can get them) and develop a more refined load once the barrel isn't so new.

Initial load testing showed no pressure signs at, no extractor marks, no flattened primers, stiff bolt lift, nothing all the way to Hornday max (54.7 grns H4831sc).

Put a DPT suppressor on it (no change in barrel length and using factory thread) and when re-zeroing. Bolt lift was noticeably stiff and the brass (new ADG brass this time too) has visible imprinting of the extractor hole - not what I would think was dangerous levels of pressure but if I was testing for loads I'd probably stop there. Velocity was within 8 FPS of the average I noted during load development when no pressure was noted. 2820 FPS with supp vs 2828 FPS without previously - which is slow compared to published velocities. Accuracy was hard to say but two final shots were touching when confirming zero after adjustment so at least on par with previous testing.
I do note the first round was noticeably stiffer to open the bolt after firing so maybe it was because of a cold clean bore shot? But the extractor marks remained identical in subsequent shots (shot 5 to confirm/adjust zero).

I am surprised a suppressor changed chamber pressure to this degree as the max chamber pressures should be reached well before the projectile gets near the suppressor. Has anyone experienced a similar thing, the difference seems considerable where as I might have expected a very slight increase.
Or is there anything in case prep/rifle prep that I might have done? One thing I am thinking is possibly I didn't clean the resizing lube off the cases well enough compared to the original loads and I had higher bolt thrust rather than chamber pressure.

Originally Posted by Makros

One thing I am thinking is possibly I didn't clean the resizing lube off the cases well enough compared to the original loads and I had higher bolt thrust rather than chamber pressure.

That’s worth investigating, also make sure the chamber is clean as lube can build up over time.

Interesting thread. No clue myself.

Have you removed the suppressor and fired the same load to see what happens?

Same load and same batch of reloads (made at the same time) I've previously fired without suppressor. I might take it to the range this weekend though an try with the suppressor and without under the exact same weather, rifle conditions. Will isolate whether it's the suppressor or if I have changed a variable that I'm not aware of.

How many rounds down the barrel? Have you cleaned it could be carbon ring my 6.5 saum was bad for that

I also thinking carbon ring give it a bloody good clean and try again

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Originally Posted by Kelton

How many rounds down the barrel? Have you cleaned it could be carbon ring my 6.5 saum was bad for that

Was cleaned with Boretech eliminator prior to going out (I could see copper and I'm roughly following the break in procedure form the manufacturer so I cleaned back to sparkling clean). Then dry patched to remove all traces of cleaner (dry patched chamber as well). First shot fired was as described above. Only fired 5 shots with suppressor on.

Total rounds down the barrel since new would be sub 50 based on the once fired brass I have (only even shot by me and with that brass)

Suppressors do not increase pressure but do increase the time/pressure curve because they slow the exit of the muzzle gases. This can show as dirty cases in semi-autos but there is usually no noticeable effect in a bolt action. What you are describing is most unusual.

Give it another scrub with eliminator focusing on the throat make 10cycles front and back threw whole barrel make sure you exit at each end before changing direction leave it to soak for 20 mins muzzle slightly down cover brush in eliminator again do another 10 cycles and clean out with a patch’s until clean . For the throat I stick the brush in and turn it making sure the brush isn’t unwinding off the rod I use the next cal up if possible . It is possible to have a ring given the round count hope you get it sorted mate .

Originally Posted by gundoc

Suppressors do not increase pressure but do increase the time/pressure curve because they slow the exit of the muzzle gases. This can show as dirty cases in semi-autos but there is usually no noticeable effect in a bolt action. What you are describing is most unusual.

My thoughts exactly - I'll do some testing this weekend and see what is going on.

Originally Posted by gundoc

Suppressors do not increase pressure but do increase the time/pressure curve because they slow the exit of the muzzle gases. This can show as dirty cases in semi-autos but there is usually no noticeable effect in a bolt action. What you are describing is most unusual.

I’ve a suppressor on a bolt action 7.62x39 and being a ‘lefty’ I’ve noticed some gas etc coming out the breech area upon firing, I just thought it was part and parcel of having a suppressed rifle.

I was using eliminator and patches came out clean but still had a carbon ring had the barrel removed to see what the issue was, it turned out to be a carbon ring.
I now use a foaming bore cleaner Gun Slick it's awesome and no more issues.
Carbon can build up very quick.

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Seems to in the high horse power 6.5s I’m still yet to find that foam cleaner I keep forgetting to be fair

It is a phenomenon that I have noticed and also talked about recently on the us Longrange hunting forum.

Also, you need to make absolutely sure that your cases and your chamber are dry of any lubricant.