7 PRC barrel slowing down?

[bonepoint]

With a 20" barrel and less than 200 rds doen the tube I'd be happy with any of those if they printed good groups. Personally I'd test a 10 shot group with 68.2gr of HD Staball and go from there.

I actually did try 69.0g of HDstaball (this time with the berger load, which is longer AOL and lower pressure, otherwise would have dropped lower) and the 3 shot group was 0.8". It was overcast and the chrono didn't work for those 3. The stiff bolt HDstaball load was with the ELDX and it was a 1.0" group, which is the worst the rifle has shot. The gun does shoot both bullets to the same POI and group sizes (just at different OALs). A few variables there and a small sample size, but the HDstaball groups just weren't showing as much promise as the other powders.

I've been seeing this in my recent build. The ejector marks show but nothing to note on the primer. There's something to that. My zermatt 300prc does the same. What did the primer look like on the round where you had some pressure and a sticky bolt? I will say your ejector marks are pretty gnarly though. Mine, you have to hold them just right in the light to pick them up.

The brass ejector marks were really wicked (with visible ridges) on the stiff bolt load (which was under book max), and the primer still looked fine. Apparently, I guess I can't rely on primers for pressure signs because I am sure I was over pressure on this one

 

[bonepoint]

most of my loads have ejector marks, but no flat primers or sticky bolt. I run a bit hotter than books which are usually a bit low anyways.

im getting 2950fps with a 168gr LRX with 65.5gr of H4831SC. Its a few grains over book max but my rifle was telling me I could keep going. at 66.5gr I started to flatten primers and the bolt got a little sticky.

The last few lots of H4831sc I have had have taken about 2g over book to match velocity and pressure (in a couple 270s and a 7RM). While probably fine, I just avoided using it because I didn't like being that much over book when the other powders tracked it well.

I am going to load up a few with H4831sc and see what I get, though.

 

[eoperator]

I would give n570 a try

 

[Warford305]

Couple things here, I'll start with 3 shots doesn't give you enough data to really put any merit into anything, so I suggest upping your sample size when trying to diagnose a difference or issue. I find 10 as suggested a lot here is pretty sufficient to start making comparisons with.


Questions -
Same powder lot?
Similar temp/conditions when testing?
Was the ammo loaded and shot soon after, or did any of it sit for a period of time before firing?
Do you use neck lube?

With 3 shot groups you can't distinguish if that powder or combo is just erratic and you caught a couple of the highs and a couple of the lows for your averages. Virgin brass is sometimes a little bit slower on first firing as some of the energy is absorbed by the case being fireformed, and the variation in case dimensions. Changing the OAL just adds another variable.


My suggestion would be to fire all the brass, properly resize it with .002" shoulder bump, do a short pressure ladder to make sure you're still safe, and test with bigger samples. You also might see a velocity increase as you approach 150-200 rounds on the barrel, but you need a good baseline to judge off of.

Running the numbers on a 175 ELD-X@2700fps vs a 165 TGK@2850fps, at 800 yards with 10mph wind the impact velocity is 20fps faster with the 165, 13" more drop on the 175, and 2.2" more wind drift with the 165. So really not much there. A 150 doing 3k adds almost 100fps to impact velocity, but almost identical wind drift as a 165. So unless you're trying to extend your effective range past 800 yards, I think the high BC outweighs the other ggoodn

 

[35WhelenAI]

most of my loads have ejector marks, but no flat primers or sticky bolt. I run a bit hotter than books which are usually a bit low anyways.

im getting 2950fps with a 168gr LRX with 65.5gr of H4831SC. Its a few grains over book max but my rifle was telling me I could keep going. at 66.5gr I started to flatten primers and the bolt got a little sticky.

BS your rifle was "telling me I could keep going". The rifle/brass is telling you something you don't want to hear. Flat primers and sticky bolt at 1 grain over the load you are referencing? One grain less hasn't fixed the issue if a sign of pressure remains.

Over book (since when are they "low anyways" in a 7 PRC?) and ejector marks, what part of that doesn't register????

 

[bonepoint]

I tried a couple loads with H4831sc. Velocity was similar to the other powders (which was better than I expected), but accuracy not as good as N560 or H1000. Yeah, I know they are only 3 shot groups, but the ELDX and EH were always around a half inch with N560 or H1000.

175 EH, 63.5g H4831sc,, 2,650 FPS - 0.9" 3 shot group. No pressure signs.
175 EH, 64.5g H4831sc,, 2,750 FPS - 1.0" 3 shot group. Very light ejector marks. Primer round, bolt lift fine.
160 AB, 65.5g H4831sc, 2,950 FPS - 1.0"

 

[171farm]

I always found it kind of counter intuitive to bring out a "long range" round and then produce rifles with 20-22" barrels. I get the point of some people wanting to shoot suppressed, but I bet the vast majority of people that are buying a rifle off the shelf are likely not throwing a suppressor on it. You would think most, like the original poster are going to be looking for speed to reach the true potential of the round. Just seems odd to me...while Im ranting, same with pencil thin barrels so many rifles come with now. I get it, there is a purpose for them, just not for everyone.

Back on topic, I think everyone has given some great advice already

 

[wind gypsy]

I always found it kind of counter intuitive to bring out a "long range" round and then produce rifles with 20-22" barrels. I get the point of some people wanting to shoot suppressed, but I bet the vast majority of people that are buying a rifle off the shelf are likely not throwing a suppressor on it. You would think most, like the original poster are going to be looking for speed to reach the true potential of the round. Just seems odd to me...while Im ranting, same with pencil thin barrels so many rifles come with now. I get it, there is a purpose for them, just not for everyone.

Back on topic, I think everyone has given some great advice already

By that logic, people should only choose cartridges with poor ballistics if they want to use a shorter barrel or lightweight rifle? Does not compute.

 

[SDHNTR]

The edges of that primer cup look undisturbed though, for whatever reason I think it's pretty common to see them pronounced on bolts with double ejectors. If that was brass from a load that was 1gr or more under bolt lift with a stable powder, I'd roll with it.

I've seen this a lot too. I'd love a logical explanation why dual ejectors leave early marks on brass.

 

[Harvey_NW]

I've seen this a lot too. I'd love a logical explanation why dual ejectors leave early marks on brass.

The only thing I can think of is maybe less individual spring resistance allowing the brass to be imprinted even at safe pressures.

 

[171farm]

By that logic, people should only choose cartridges with poor ballistics if they want to use a shorter barrel or lightweight rifle? Does not compute.

Thats not what I said or at the very least not what I meant. Im just saying if your looking for speed going with a shorter barrel typically doesnt make sense. Im pretty sure its widely believed the PRC was designed around a 24" barrel or at the very least thats the length velocities are stated for. So if you have a 20" you should expect to get bellow typically states velocites

 

[wind gypsy]

Thats not what I said or at the very least not what I meant. Im just saying if your looking for speed going with a shorter barrel typically doesnt make sense.

It makes perfect sense. Why cant someone want both a certain speed and a certain barrel length and then pick the appropriate cartridge to achieve that goal?

Im pretty sure its widely believed the PRC was designed around a 24" barrel or at the very least thats the length velocities are stated for.

Cartridges aren't designed around barrel lengths far as I can tell. 24" is a common barrel length used by ammo manufacturers for most any hunting ammo. I don't recall seeing any bolt action centric hunting ammo listed with their baseline velocities from a 22" or 20" barrel.

So if you have a 20" you should expect to get bellow typically states velocites

Of course.

 

[brushape]

You probably have a carbon ring or something like that. On the people saying you need to shoot more than a 3 shot group that’s just dumb. If your rifle is slow in the first 3 shots it’s not going to get faster after that and if it does your es is going to trash and unusable anyway. Same thing goes for if you’re loading for groups and your first 2 shots in a powder charge are 3” apart you can shoot as many as you want after and you aren’t going to save the group


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[Shooter Mike]

I always found it kind of counter intuitive to bring out a "long range" round and then produce rifles with 20-22" barrels. I get the point of some people wanting to shoot suppressed, but I bet the vast majority of people that are buying a rifle off the shelf are likely not throwing a suppressor on it. You would think most, like the original poster are going to be looking for speed to reach the true potential of the round. Just seems odd to me...while Im ranting, same with pencil thin barrels so many rifles come with now. I get it, there is a purpose for them, just not for everyone.

Back on topic, I think everyone has given some great advice already

I see your point, and mostly agree. I bought my 7 PRC with a 22” barrel for the sole purpose of shooting it suppressed. I couldn’t imagine hunting without a suppressor now. Speed has its place, but if I can preserve what’s left of my hearing while shooting, that’s something I won’t pass on.


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[Harvey_NW]

You probably have a carbon ring or something like that. On the people saying you need to shoot more than a 3 shot group that’s just dumb. If your rifle is slow in the first 3 shots it’s not going to get faster after that and if it does your es is going to trash and unusable anyway. Same thing goes for if you’re loading for groups and your first 2 shots in a powder charge are 3” apart you can shoot as many as you want after and you aren’t going to save the group

And conversely, which is the crux of suggesting larger sample sizes, if your first 3 are .3", that could still open up to almost 2" with more shots. Yes, 3 shots can tell you how bad it is. But that has little to no statistical validity to how good or consistent it is.

 

[Vern400]

A few things are puzzling me, so looking for some input.

I bought a Fierce 7 PRC with a 20" barrel. It has shot everything very accurately, but it is very slow and seems to be getting SLOWER with more rounds down the barrel.

Factory Hornaday Precision ammo was right at 2,700 fps, but did shoot <0.5" groups as I was breaking it in.

I have tried a several loads of 175 ELDX and 175 Berger EHs with N560, H1000, Retumbo, and HD Staball. Everything has been under an inch at 100 and may were under 1/2" (3 shot groups).

Here are a few load results that have me scratching my head.

Less than 10 rounds down the barrel.....
175 ELDX, 64.5g N560, 3.340" = 2,770 FPS with 0.6" group

60 rounds down the barrel......
175 ELDX, 66.0g N560, 3.300" = 2,700 FPS

More rounds down the barrel, 1.5g more powder, shorter OAL, and slower velocity?! This pic below is of my brass of that last load. I am at book max and have ejector marks, but primer looks fine. I am hesitant to try more powder in that situation, especially since velocity didn't go up on the last jump.

Best load.....
175 Berger, 66.5g N560, 3.365" = 2,700 FPS with 0.3" group

A few other pieces of info worth noting.
- I pushed a few other powders (retumbo and HD staball) higher and did get a stiff bolt at around 2,800-2,850 fps. I was at or below book with both powders..
- All loads are with virgin Peterson brass.
- I wondered if my cheaper pro chrono was acting up. I shot it alongside a buddy's magnetospeed (on a different rifle since the PRC has a suppressor) and it tracked well.


While the gun is very accurate, I have hard time accepting the 2,700 FPS for my long range gun.
Should I just accept it and move forward with my Berger load?
Would you feel comfortable trying more hotter loads with brass and primers below being at max charge but very low velocity?
Or should be try a little lighter bullets to try to get the velocity up?



View attachment 712605

I am not buying your story that you're over pressure yet. Your primer looks good. The outside radius looks very similar to that of a new primer. Some are harder than others. I use Federal so that is a qualifier for my statements.

I can take cartridges from a large batch that I've been shooting for 10 years, and either leave oil in the chamber, or leave a very thin film of Case lube on the cartridges... And I could post a picture here, and I'd have armchair quarterbacks lined up taking a turn to call me an idiot for shooting them. They extrude into the extractor hole because they can't grip the chamber.

I can depend on my first two rounds after cleaning to leave a light extractor mark, even if I swab out the chamber. And they will go 3/4 inch high left. Third shot onward is point - on.

They're perfectly normal cartridges, well within published recipes, and I shoot them successfully in two different rifles.

Because you have extractor marks, but your primers look normal, it looks like a condition I can create in 5 seconds with safe cartridges.

If you wipe a few cases down with alcohol, and also swab your chamber, and get the same result? Then yes you may be approaching or exceeding Max pressure.

I was so worried when this happened to me!
I'm telling you exactly what Sierra tech support told me. In my case, they were correct.

Using harder brass, and small primers is not a solution I am comfortable with. Even in 308 and 6.5. Some people do it. I don't think using Lapua brass gives me a free pass to go over 62,000 PSI.

But you might consider trying a few samples of different head stamp brass. I don't know if Peterson brass is soft like Federal brass tends to be. But if they are the outlier and their brass is very soft it could be contributing to the extractor marks. It's an outside chance but it's real.

308, 6.5 CM, 338 Lapua, 50 BMG all run in the 2700 to 2850 fps with standard ammo. Somebody will probably correct me by 50 fps. Whatever.
Even if your rifle is tapped out at 2700 FPS with a shortened barrel, it's not incompetent at long range. But I'm not convinced you're tapped out.

Good luck!