Suppressors cause Increased Chamber Pressure?

[ccmdfd]

If you have a decent set of dial calipers, measure case diameter .200" above the base- just above the extractor groove, and see if there is a difference between the two.

0.467 unsurpressed, 0.468 suppressed

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0.467 unsurpressed, 0.468 suppressed

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Suppressed is definitely seeing more pressure. From here, it's hard to say if it is the load or the rifle. Do you have any other ammo to try?

[Clusterfrack]

Looking at the shape of the primers and the lack of ejector/extractor wipe (mark), I don't see pressure signs.

It always pays to be careful, but I think this thread is headed down the wrong rabbithole.

[5pins]

Great question, and excellent choice in suppressor. My suggestion is to lube your lugs with a little grease and carry on.

@JCL may have a more technical answer?

This

Grease the bolt lugs and it should help.

This seems to happen in rifles with tight chambers and hot loads. My theory is that the suppressor keeps the chamber presser up longer and doesn't allow the brass to shrink down as much after firing. I don't know if that is true or not just my guess.

[Skinner Precision, LLC]

The primers are not flattening but you are getting a little bit of primer flow into the firing channel in both suppressed and unsupressed scenarios. That tells me that the load is a little on the warm side, or the chamber is a little tight.

If you have a decent set of dial calipers, measure case diameter .200" above the base- just above the extractor groove, and see if there is a difference between the two.

Seconds Count-

When evaluating primers you have to be careful to consider what is the "normal condition". I do not see pressure when I look at the primer flow, I see an oversized firing pin channel in the bolt. If the OP shot a lot of small rifle primer brass reloads (such as Lapua or Peterson brass) they are more likely to have problems but with a bolt with a big FP opening, it is easier to have primers flow ( and pierce). This isn't a pressure thing , the primer is flowing into the over large opening in the bolt then gets sheared off. The answer is to have someone like Greg Tannel "bush" the bolt- installing a hardened bushing with a smaller diameter opening and turning down the firing pin to a smaller OD.


In regards to measuring brass for pressure signs a caliper isn't discrete enough, you need a tenths reading (.0001" resolution) micrometer and brass that is virgin (no work hardening) and then measuring is still a poor estimation of pressure but like reading primers, the average person has few options...

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Seconds Count-

When evaluating primers you have to be careful to consider what is the "normal condition". I do not see pressure when I look at the primer flow, I see an oversized firing pin channel in the bolt. If the OP shot a lot of small rifle primer brass reloads (such as Lapua or Peterson brass) they are more likely to have problems but with a bolt with a big FP opening, it is easier to have primers flow ( and pierce). This isn't a pressure thing , the primer is flowing into the over large opening in the bolt then gets sheared off. The answer is to have someone like Greg Tannel "bush" the bolt- installing a hardened bushing with a smaller diameter opening and turning down the firing pin to a smaller OD.


In regards to measuring brass for pressure signs a caliper isn't discrete enough, you need a tenths reading (.0001" resolution) micrometer and brass that is virgin (no work hardening) and then measuring is still a poor estimation of pressure but like reading primers, the average person has few options...

Primer flow into the firing pin channel is an indication of pressure, but not necessarily dangerous levels.

Your experience with measuring brass is different than mine, and can indicate chamber issues, causing heavy bolt lift.

[Skinner Precision, LLC]

Primer flow into the firing pin channel is an indication of pressure, but not necessarily dangerous levels.

Your experience with measuring brass is different than mine, and can indicate chamber issues, causing heavy bolt lift.

NO, Primer Flow CAN be a pressure problem- but it is not absolute. Re-look at the pictures with the radius of the primer, it appears unaffected. If there was no radius visible in the picture, I would opinion differently vis-à-vis the cause, but with factory ammo doing it and the radius clearly visible, I stand by my statement of knowing what is "normal" for that gun is critical, and that overpressure shouldn't be declared from the flow alone. Conversely, if the OP had posted a picture of deprimed a primer and it looked like a top-hat from excessive flattening and showed flow (or piercing), that would be an obvious clue of pressure.

There is a reason knowledgeable people do not build/recommend building 6.5 creedmore AR-10's with DPMS 308 bolts heads with their gapping firing pin holes and that JP/Rubber City/Aero all advertise small firing pin bolts- completely safe, within SAAMI pressure ammo, caused problems with large frame builds. A decade before that example became common knowledge, people discovered that with building non-308's on all the cheap FN (USRA) SPR short action model 70's that flooded the market for a while, with a huge firing pin openings, could cause problems (especially when venturing past .308 chamberings). A decade before that, it was the sudden popularity with 6mm br's and all the people with non-custom ( customs typically had small firing pins back then and now) actions discovering sometimes (not all the time) needing to get their bolts bushed.

Higher Pressure cartridges (still within SAAMI SPEC for the round) and small rifle primers/.473 casehead combos both show the flow faster but the amount of the primer that is unsupported (how oversize firing pin hole) is often the key variable into if a primer will flow and/or pierce. I personally had a new 1990's Model 70 .308 varmint I bought to build into a NRA XTC rifle (post 64 , short action, factory wilson barrel) that would horribly crater primers with all kinds of factory ammo and known good reloads, the headspace was correct and the ammo fit the chamber and the chamber wasn't improperly sized from the factory. The FP hole was the problem. Similarly I built a LR hunting rifle (7 mm Rem mag) on a model 70/Manners/Krieger for a customer who had/has a factory 40X 1000 yard prone gun in the same caliber. He had horrible flow and pierced primers from the get go with the factory ammo/his handloads. The chamber wasn't a problem when checked and again all his problems instantly went away when the firing pin hole (IRC, his pin gauged at .079 prior to surgery) was bushed by Greg Tannel, who did the job cheaper than I would be willing to tackle it for a good friend/customer for.

1.You have to know what the "normal condition" is for rifle before reading pressure signs in the tea leaves that are contained a spent piece of brass. 2. Ejector marks are one of the few "sure" sign of pressure , if you see them STOP and go back.

Some brass (a combination of some brands metallurgy and the cross-section of some cartidges) can mask pressure signs WAY WAY past 65KPSI and some combinations cause problems before "safe" sammi spec pressures are exceeded.


OP, like @Clusterfrack, I agree that it pays to be careful. I recommend buying several other brands factory ammo and shooting through your rifle, unsuppressed, to learn about what is "normal" before making any estimations as to excessive pressure signs. I also agree with his recommendation to lube the bolt lugs but please do not lube ammo...

[Skinner Precision, LLC]

@ccmdfd
I am also not discounting measuring case head expansion as an estimation of pressure, but the devil is in the details. Long ago, there were published reloading manuals that indicated an observed .0002" difference between safe and overpressure and that a tenths reading micrometer was the required instrument and that work hardened brass behaved differently. A caliper is the wrong piece of meteorology for this exercise. I do not want to insult your measuring abilities - either skill or instruments available.

Before I tried to determine pressure differential between suppressed and unsuppressed by case head expansion measurements, I would want to measure enough brass, unsuppressed, to be confident that I was consistently and accurately measuring brass (multiple measurements on the same piece of brass and multiple pieces of brass) to gain awareness of how much my measurements varied (and to detect if there was something else going on that I was picking up in my measurements), while not forgetting the necessary resolution of the measurements.

[ccmdfd]

@ccmdfd
I am also not discounting measuring case head expansion as an estimation of pressure, but the devil is in the details. Long ago, there were published reloading manuals that indicated an observed .0002" difference between safe and overpressure and that a tenths reading micrometer was the required instrument and that work hardened brass behaved differently. A caliper is the wrong piece of meteorology for this exercise. I do not want to insult your measuring abilities - either skill or instruments available.

Before I tried to determine pressure differential between suppressed and unsuppressed by case head expansion measurements, I would want to measure enough brass, unsuppressed, to be confident that I was consistently and accurately measuring brass (multiple measurements on the same piece of brass and multiple pieces of brass) to gain awareness of how much my measurements varied (and to detect if there was something else going on that I was picking up in my measurements), while not forgetting the necessary resolution of the measurements.

No offense taken whatsoever!

While I've been shooting for most of my life, I am an absolute newbie in terms of reloading, taking measurements, etc.

I'm amazed at watching these videos where people talk about how 0.003 inches in seating depth can make a huge difference in performance. Yet I can easily get my calipers to change by 0.003 just based upon how I'm holding my mouth (as Grandma used to say).

So I don't put a whole lot of faith in a measurement difference of 0.001 in when I measured the case head/body.

I appreciate your thoughtful and detailed responses to this thread.

[Skinner Precision, LLC]

No offense taken whatsoever!

While I've been shooting for most of my life, I am an absolute newbie in terms of reloading, taking measurements, etc.

I'm amazed at watching these videos where people talk about how 0.003 inches in seating depth can make a huge difference in performance. Yet I can easily get my calipers to change by 0.003 just based upon how I'm holding my mouth (as Grandma used to say).

So I don't put a whole lot of faith in a measurement difference of 0.001 in when I measured the case head/body.

I appreciate your thoughtful and detailed responses to this thread.

Thank you for your grace when reading my posts. In regards to the internet it is tricky to separate the wheat from chaff as everybody has a rifle that shoots 1/4 MOA all day long, when they "do their part" (as witnessed by a single 3 shot wallet group they shot years ago) and every gunsmith can cut a chamber with .0001 runout on their lathe with Chinese spindle bearings in their non-climate controlled garage...

When I saw the. 001 measurement differential I had a couple thoughts beyond my previous statements on measuring case head expansion like

1. He is probably using "hobby grade" calipers, good (expensive)calipers have parallel jaws , cheap ones often have manufacturing shortcuts that result in a definite lack of parallelism. A sleazy way to check parallelism of said hobby grade calipers is to hold them up to the light and look for a gap...
2. Measuring a soft, malleable item with a steel jaw takes some finese to get good repeatable measurements
3. While I strongly believe that a chamber, cut by either single pointing (boring bar) or form tool (chamber reamer) will ALWAYS be round (albeit oversize if the application is f'd up), that doesn't mean that brass fired in that round chamber, when measured, doesn't exhibit sometime exibit measurable eccentricity.