7 PRC barrel slowing down?

[bonepoint]

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?

 

[jimh406]

If you have the more recent Precision Hunter ammo, most are only getting around 2800, so 2700 with that ammo should be pretty good. Their original spec was 3000, I believe which should put you around 2900 with the shorter barrel.

Otherwise, what primers are you using?

 

[Harvey_NW]

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.

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.

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.

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?

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 gains.

 

[bonepoint]

Otherwise, what primers are you using?

Win LR Mag

Harvey-NW,

Questions -
Same powder lot? Yes
Similar temp/conditions when testing? Similar, but a little warmer with the later load. Have to go back and look at my notes, but within 20 degrees.
Was the ammo loaded and shot soon after, or did any of it sit for a period of time before firing? No sitting
Do you use neck lube? No.

I bought 250 pieces of brass. I do need to get my neck dies ordered for resizing

 

[DiabeticKripple]

have you cleaned the gun down to bare metal since the break in?

 

[jfk69]

I guess I don’t see 2700 fps out of a 20” barrel as bad at all. And while HarveyNW is correct on the number of shots for groups, I’m betting it will throw 10 down range pretty accurately. Accuracy>Velocity.

 

[Harvey_NW]

I bought 250 pieces of brass. I do need to get my neck FL die ordered for resizing

Fixed it for you.

 

[doverpack12]

It’s a short barrel. Nothing wrong with 175 good bc bullet at 2700 fps unless you are looking to shoot very very far.

All powders listed are slow burning which means long barrel. If you were set on gaining velocity you could try the next tier faster powders.
180 Berger with H4831SC is a classic 7 RM combo in longer barrels, don’t be afraid to go on the fast side of H4831SC or try that powder.

 

[Fireflyfishing]

Are you crimping your loads? If yes, try without a crimp.

 

[bonepoint]

have you cleaned the gun down to bare metal since the break in?

Yes, Wiped out oil from barrel and action out of the box. Cleaned barrel once during break in and didn't get much copper out of the barrel. Shot the same before and after.

Are you crimping your loads? If yes, try without a crimp.

No crimp.

 

[bonepoint]

Fixed it for you.

I meant neck bushings to go with the Hornady Match dies.

 

[Ucsdryder]

Those are some mean ejector marks.

 

[bonepoint]

I guess I don’t see 2700 fps out of a 20” barrel as bad at all. And while HarveyNW is correct on the number of shots for groups, I’m betting it will throw 10 down range pretty accurately. Accuracy>Velocity.

It’s a short barrel. Nothing wrong with 175 good bc bullet at 2700 fps unless you are looking to shoot very very far.

All powders listed are slow burning which means long barrel. If you were set on gaining velocity you could try the next tier faster powders.
180 Berger with H4831SC is a classic 7 RM combo in longer barrels, don’t be afraid to go on the fast side of H4831SC or try that powder.

I should probably just run with that since it does shoot great, I was just disappointed with the velocity. I probably just tinker with it too much. I do have H4831sc, but haven't tried that yet since I was already battling lower speeds, and usually run even lower with that powder. I may try a few and see if I do get better velocity out of the shorter barrel.

H1000 shot the tightest group (175 berger, 3.365", 67.0g H1000), but it was only around 2,650 FPS (below). Groups opened up as I added a grain more.


The N560 load (175 berger, 66.5g N560, 3.365") shot about the same, and was around 2,730 FPS (below). I got brass marks and a little bigger groups (0.8") when I went with a shorter OAL, so this is probably about max. Maybe will just load up 10 of these and see how it goes, and accept the velocity if it shoots similar. Sounds like people don't think the velocity is that slow for a 20" barrel.

I may still try running H4831sc up to max to experiment, but would be shocked if it beat N560 on speed, and it would be harder to do any better on accuracy.

 

[Harvey_NW]

Those are some mean ejector marks.

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.

 

[bonepoint]

Those are some mean ejector marks.

I know, which is why I didn't want to go any higher. But velocity was low and primers were not very flat, so wanted other opinions.

 

[bonepoint]

Those are some mean ejector marks.

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 never ran N560 up to a stiff bolt. The load that shot the best had very faint ejector marks. I would guess I am 1 to 1.5g away from a stiff bolt with 66.5 of N560, which is already .5 over book max.

I hit a stiff bolt with two loads.
70g Retumbo, 175 berger, 3.365" at 2,780 FPS (book max)
69.5g HDstaball, 175 ELDX, 3.340" at 2,830 FPS (1.5g under book max)

So, for the most part, it appears to track pressure pretty close to the book maxes on the powders I tried. Just the velocities didn't track with it.

 

[Jason Snyder]

I’d try 4831 before I did anything else.

 

[Harvey_NW]

I never ran N560 up to a stiff bolt. The load that shot the best had very faint ejector marks. I would guess I am 1 to 1.5g away from a stiff bolt with 66.5 of N560, which is already .5 over book max.

I hit a stiff bolt with two loads.
70g Retumbo, 175 berger, 3.365" at 2,780 FPS (book max)
69.5g HDstaball, 175 ELDX, 3.340" at 2,830 FPS (1.5g under book max)

So, for the most part, it appears to track pressure pretty close to the book maxes on the powders I tried. Just the velocities didn't track with it.

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.

 

[SloppyJ]

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.

 

[DiabeticKripple]

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.