7mm PRC Pressure Signs?

[Harvey_NW]

IMO, you're just looking at crappy Hornady brass. Nothing more.

I agree, sometimes on soft brass the bolt face just shows up. My 6 Creed would put an ejector mark on a Hornady case head at a starting charge. I didn't care about brass life so I went to a middle of the road charge and got 6 firings before the primer pockets started to loosen up.

That's a bummer there's that much expansion at the web! Seems like you're already into clicker territory with that brass just off the factory firing. I wouldn't be surprised if it will be prone to sticky extraction on every firing forward now.

Didn't clickers come from having the chamber:brass tolerances too tight though? That's the way I understood it with the Sherman Max BS I ended up in, with his new corrected "B reamers" blaming it on ADG and Gunwerks..

My Sherman brass looked identical to that first firing, and I learned that using calipers is a rudimentary form of measurement for web growth. IIRC it needs to be taken with a micrometer using a .200" spacer block, and is still not 100% reliable. I think that's from the brass adhering to the chamber walls on firing, and has less to do with case head separation, but I could be wrong. I imagine if you're measuring and dial a consistent .002" shoulder bump, you'll get an average Hornady number of firings off that brass.

Interested to hear feedback on the web growth and if you develop clickers or not.

 

[wind gypsy]

Didn't clickers come from having the chamber:brass tolerances too tight though? That's the way I understood it with the Sherman Max BS I ended up in, with his new corrected "B reamers" blaming it on ADG and Gunwerks..

I'm no authority on it, just have done some digging around. In regards to the ideas of clearance - after it came up in relation to the PRC SAAMI specs i looked at some other magnum cartridge SAAMI specs and saw that the spec'd clearance @ the web between chamber and brass was about the same. Just spitballin but it seems like some cases just tend to expand too much at the web and cause issues. I know with 300 Norma clickers are likely if using Norma brass and peterson or lapua brass is less likely to grow at the web and develop clickers.

So, it seems like a combination of having a case that resists expanding at the web and a chamber with a little extra clearance are the way but that's not always easy..

My Sherman brass looked identical to that first firing, and I learned that using calipers is a rudimentary form of measurement for web growth. IIRC it needs to be taken with a micrometer using a .200" spacer block, and is still not 100% reliable. I think that's from the brass adhering to the chamber walls on firing, and has less to do with case head separation, but I could be wrong. I imagine if you're measuring and dial a consistent .002" shoulder bump, you'll get an average Hornady number of firings off that brass.


Interested to hear feedback on the web growth and if you develop clickers or not.

Sounds like he already had heavy bolt lift and if that ring on the web is from being tight in the chamber i would have lumped that in with "clicker" issues already on first firing.

 

[Harvey_NW]

I'm no authority on it, just have done some digging around. In regards to the ideas of clearance - after it came up in relation to the PRC SAAMI specs i looked at some other magnum cartridge SAAMI specs and saw that the spec'd clearance @ the web between chamber and brass was about the same. Just spitballin but it seems like some cases just tend to expand too much at the web and cause issues. I know with 300 Norma clickers are likely if using Norma brass and peterson or lapua brass is less likely to grow at the web and develop clickers.

So, it seems like a combination of having a case that resists expanding at the web and a chamber with a little extra clearance are the way but that's not always easy..



Sounds like he already had heavy bolt lift and if that ring on the web is from being tight in the chamber i would have lumped that in with "clicker" issues already on first firing.

No disagreement. I know there was some speculation that the 7 would probably follow suit with clickers, but I thought that had been brought up and addressed during design. OP did mention that the bolt lift seemed to disappear after a few rounds, and that sizing them shrunk the web measurement .003". Interested in the outcome.

 

[35WhelenAI]

In addition to what has been shared so far. What did primers look like in the once fired cases? Are new primers seating with good resistance or going in loose?

 

[TxLite]

In addition to what has been shared so far. What did primers look like in the once fired cases? Are new primers seating with good resistance or going in loose?

Fired primers had slight cratering but edges were still round. Primer pockets seem ok. Interested to see if CCI 250’s crater as well next time I can get out and shoot it.

 

[35WhelenAI]

Apologies I missed the info already posted about primer appearance. Rounded edges are a positive sign of lower pressure vs higher pressure. Tight pockets are a positive sign as well.

If you don't feel anything inside the case with a paper clip, the outer ring isn't incipient case head separation. It will show up inside the case before outward visual clues are seen or at the same time in my experience. Pressure ring and case separation are two different things.

With respect to case head measurements, a blade micrometer that measures to .xxxx or 4 decimal places is the tool for the job. A dial that measures to .xxx or 3 places is not precise enough as was shared in a post above. 0004-.0006 is considered a safe range but do not exceed.

The numbers you shared are over on case head expansion when considering the amounts if you added a fourth decimal place, but is the measurement meaningful with the tool you used? Not saying it is or isn't, asking the collective as my opinion and experience is but one person.

The case web is a pretty generic area of the cartridge head. The measurements need to be done in front of the extractor groove, and indexed on each cartridge and kept track of. Meaning, you have to take the measurement as close to the exact same place as possible with the unfired brass and the once fired brass. Can't take a case, measure it, then fire it and just measure. Gotta be the same two points of contact, on the same case for the blade micrometer. And goes without saying, each case is a different rule unto itself so the more cases you can do to establish a trend the better.

I did some load development with a powder there was no data for the bullet I was using in one of my rifles. Case head expansion was helpful to working it up, and coincided with other (lack of) traditional signs so the added point of data was useful for feeling good about the load.

 

[TxLite]

Apologies I missed the info already posted about primer appearance. Rounded edges are a positive sign of lower pressure vs higher pressure. Tight pockets are a positive sign as well.

If you don't feel anything inside the case with a paper clip, the outer ring isn't incipient case head separation. It will show up inside the case before outward visual clues are seen or at the same time in my experience. Pressure ring and case separation are two different things.

With respect to case head measurements, a blade micrometer that measures to .xxxx or 4 decimal places is the tool for the job. A dial that measures to .xxx or 3 places is not precise enough as was shared in a post above. 0004-.0006 is considered a safe range but do not exceed.

The numbers you shared are over on case head expansion when considering the amounts if you added a fourth decimal place, but is the measurement meaningful with the tool you used? Not saying it is or isn't, asking the collective as my opinion and experience is but one person.

The case web is a pretty generic area of the cartridge head. The measurements need to be done in front of the extractor groove, and indexed on each cartridge and kept track of. Meaning, you have to take the measurement as close to the exact same place as possible with the unfired brass and the once fired brass. Can't take a case measure it, then fire it and just measure. Gotta be the same two points of contact, on the same case for the blade micrometer. And goes without saying, each case is a different rule unto itself so the more cases you can do to establish a trend the better.

I did some load development with a powder there was no data for the bullet I was using in one of my rifles. Case head expansion was helpful to working it up, and coincided with other (lack of) traditional signs so I the added point of data was very useful for feeling good about the load.

Thanks for the detailed response. I plan on starting low with these using H1000, CCI 250, and 175 EH and monitoring closely for excessive pressure signs.

 

[TxLite]

Feeding and chambering fine. Planning to run it Sunday (weather and wife permitting)

 

[TxLite]

Well, velocities were pretty low today, but everything went fine as far as pressure is concerned. SD’s were pretty terrible (maybe an issue with incomplete powder burn in the 18” barrel?). Had some decent groups. The last two groups (68.5 and 69) showed a some potential for accuracy, but I think overall there’s a lot left to be desired in the velocity department. Primers still looked good so I plan on continuing forward with the H1000 and 175 EH. Total round count is 73.

 

[35WhelenAI]

How much room left in the case to go up in charge weight with H1000 from your 69 gr charge and the long 175 gr bullet?

 

[TxLite]

How much room left in the case to go up in charge weight with H1000 from your 69 gr charge and the long 175 gr bullet?

I am seating at the bt junction to maximize cass capacity. Not a ton of room left based on how it sounds when I shake the case, but still enough to be worth exploring to eek out a little more velocity. Berger showed a max charge of 70.6gr @ 104% capacity but I am seated roughly .02 longer.

 

[Bfish22]

Feeding and chambering fine. Planning to run it Sunday (weather and wife permitting)

Do you know how far from the lands you’re at with the 175’s with the 3.296 coal? Saami freebore reamer? Thanks

 

[JjamesIII]

Finally getting a chance to sit down and really look at some of the the brass from my 7mm PRC and trying to figure out what I’m looking at. This is all once fired factory ELDX in a carbon six Tikka prefit. None of these pieces have been run through a sizing die. No primers were blown, just decapped tonight. Several pieces have ejector marks. A few had sticky bolt lift. All pieces have a ring around them that appears to be the beginnings of case head separation. I ran a paper clip through several cases and cannot feel the ring from the inside. Is this overpressured factory ammo? Sloppy chamber? What am I looking at?View attachment 510387View attachment 510388View attachment 510390

 

[JjamesIII]

I am seating at the bt junction to maximize cass capacity. Not a ton of room left based on how it sounds when I shake the case, but still enough to be worth exploring to eek out a little more velocity. Berger showed a max charge of 70.6gr @ 104% capacity but I am seated roughly .02 longer.

The sticky bolt lift is concerning- that’s my primary indicator of how hot I’m loading, blown primers or loose pockets on subsequent reloads. Ejector marks can be seen on crap brass at times. I’ve had some brass get marked up with even mild loads. Have you put a reliable chrono on your rifle? If you’re getting magically high velocities, that could be another indicator of redlining the gun.

 

[TxLite]

The sticky bolt lift is concerning- that’s my primary indicator of how hot I’m loading, blown primers or loose pockets on subsequent reloads. Ejector marks can be seen on crap brass at times. I’ve had some brass get marked up with even mild loads. Have you put a reliable chrono on your rifle? If you’re getting magically high velocities, that could be another indicator of redlining the gun.

That was with the factory ammo. I haven’t seen any pressure signs with handloads yet. I’m using a magnetospeed v3 that has been accurate for me on other rifles. For what it’s worth, I also shot some factory ammo yesterday and did not see and sticky bolt lift like I did the first day.

 

[TxLite]

Do you know how far from the lands you’re at with the 175’s with the 3.296 coal? Saami freebore reamer? Thanks

Saami reamer as far as I know. I half ass checked and came up with 2.6585. This puts me jumping about .1055. Factory eldx was around .80.

 

[35WhelenAI]

You are going about this the correct way. For whatever reason the factory loads were pushing/at/over the limit in your gun.

For grins, what is the factory advertised velocity with the length of barrel you have? Or as close to it as you can extrapolate?

Velocity equals pressure, there's no free lunch on that as we all know.

 

[TxLite]

what is the factory advertised velocity with the length of barrel you have?

Hornady advertises 3000fps in a 24” barrel so I’m running about 240fps slower as my 18” barrel is starting to speed up. Comes out to 40 fps/inch. According to Berger’s load data I am running about 250fps slower than they list with a 26”, which comes out to around 31fps/inch.

I’m hoping SD’s calm down with some additional case fill, but I’m wondering if I will need to end up having to switch to something a little faster burning to get 100% powder burn in the 18”. I’ll see how things go with this next round of testing.

 

[35WhelenAI]

H1000 is a bit of a holy grail powder for that cartridge with heavy bullets. However you're on the right track to try something a tad quicker if you can't get where you think you should be.

 

[TxLite]

H1000 is a bit of a holy grail powder for that cartridge with heavy bullets. However you're on the right track to try something a tad quicker if you can't get where you think you should be.

I generally like to have an SD around 10 or under. Most loads I tried were in the 20fps range, including the factory 175. We’ll see what happens next go around. I’d saved several pounds of it specifically for this rifle so I’m hoping I can get it to work. I got on GRT and played around with the 7 Blaser mag case dimensions to have it more closely resemble the 7 PRC and it showed roughly 94% powder burn in the 18” barrel.