Pressure Signs

  • First post for me, but i've been lurking around for the past 6 months or so. I've learned a ton from the articles section here. I started reloading pistol cartridges about 1.5 years ago but have just recently started reloading for a 6.5 PRC. It's a barreled action from TS Customs in a MPA ultra lite chassis. Benchmark cf barrel and Impact action.

    I started out using Retumbo and 140 hybrids with ADG brass. I had a load that was shooting really well within 50 rounds. at around 150 rounds it was no longer shooting well. ES was almost 100 fps with average of 2850 fps. I still don't really know why that was, but attributed it to barrel speeding up and retumbo being too slow of a powder.

    I decided to try H4831sc. I settled on a load that was shooting awesome at 2940. 54.8 grains. I shot a local practice prs match which was 100 rounds and it shot awesome all day. I took it out to Montana on a elk scouting trip and shot 50 rounds at prairie dogs. Even shot one at 650 yards on the second shot which I thought was pretty good! It shot awesome until the last 4 rounds all of a sudden I got heavy bolt lift. I put it away and brought it home and cleaned it really well with wipe out patch out. I was at 350 rounds and it hadn't been cleaned since 125 rounds.

    I shot again yesterday and had slight heavy bolt lift on some of the rounds and if you look you can find a faint ejector mark on some of the cases. My average velocity yesterday was 2970. ES of 25, but SD of 9 over the course of 25 shots. I figured that was acceptable for a sample size that large, and it's shooting very accurate. My first three shots on freshly painted steel at 600 yards were 3".

    I guess i'm just kind of frustrated because the gun shoots so dang well, but now I suppose I need to go back to the drawing board on a load again. I will say it seems like the gun shoots anything pretty well though. Hornady Match ammo shoots very similar to my reloads as far as accuracy, the small amount I have tested it.

    When I did my original ladder test for this load I didn't experience pressure signs until 56.0 grains so it seems odd that I would have pressure at 54.8. Is there carbon buildup somewhere that I didn't clean well enough? Is H4831sc that temp sensitive from 60 degrees (when I did load development) to 80 degrees? Another thing I thought about that would take further testing would be how long the cartridge is in the chamber. Might explain why the slight heavy bolt lift is not 100% of the time. Or do I just need to drop down to a lower node that has more forgiveness?

    Thanks for any experience you may be able to share

  • Hello and welcome.

    It appears you jumped from the slow side of the burn characteristic straight to the fast side. Optimal powder for the 6.5PRC is in the H1000 category. You might give that a try.

    You're correct in that you'll always want to run the low node. Pushing for velocity that isn't supported by the componentry is generally not a good idea. :)

  • Yeah H1000 has eluded me for the past 6 months. I do have Accurate Magpro which I know is close to the same burn rate, but I read that it's really temperature sensitive. I've never used it so I don't know.

  • @orkan Assuming I don't have access to H1000 and am sticking with H4831sc

    Should I account for as much barrel speed up in the 500-1000 rounds down the barrel as the first 500?

    I'll do another OCW test and want to make sure I drop low enough.

  • @hbird said in Pressure Signs:

    Should I account for as much barrel speed up in the 500-1000 rounds down the barrel as the first 500?

    No. Typically the 250rnd mark will put on most of what it will do. By 400rnds, you should have it totally settled in... even in a squirrely barrel.

  • @orkan Thank You!

  • I figured I would update this. I finally got around to shooting again. I got 5 pounds of H1000 finally so I did a pressure ladder with it. I’m shooting a local match next weekend and I’m thinking I’ll go in at 53.5 of H4831sc. I didn’t experience any pressure except for 58 grains of H1000. I did have one bad shot which was that low flyer on 53.2.

    I don’t really have time to do a full ocw test on the H1000 right now.

    I read the load development article again and if I would’ve backed down 2 grains from pressure and made that my ceiling I would likely be in this node. It’s pretty easy to overlook that kind of stuff when it’s all new. Over all I’m pretty happy with the results here.

    Anything I’m missing??
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  • Certainly demonstrates the value of the 6.5PRC. Super forgiving cartridge to work with.