Pressure



  • I’ve been loading my Dasher with lapua brass, 32.2 varget, cci 450 and smk 107.

    Been shooting this since barrel break in. This load has worked nicely for it. Made 3 matches this season and at the last one, half way through the match she started shooting all over the place. First thing I checked over the lunch break was my rings and mount and the action screws and the can. All good. There are 650 rounds through it and 550 since cleaned. The week before the match I checked zero and was all good. When I got home after the match I put it to paper with the Labradar to check speeds my groups opened up from .18 to over 2”. The Labradar didn’t want to work that day so I couldn’t tell what the speeds were with my usual load.

    Fast forward to yesterday. Had my son take it out when he and Travis went out to check rifles and sent him with my usual load and also loaded up some 32.6-32.8-33.0-33.2 and 33.4 to try out. I also went from 1.777 ogive length to 1.806 thinking maybe a little throat erosion maybe.

    Speeds stayed consistent to what they were usually but was still throwing the groups opened with the 32.2 I’ve been using, Drew said the 33.2 and 33.4 were good triangle groups and just need to set length to tighten them up. But.....the just of my question....

    They noticed pressure at the 32.8 and 33.0 loads, however at the 33.2 and 33.4 the pressure signs went away. Now, I’m no expert in the least, but, in my mind if you get pressure signs at a lower charge, wouldn’t those pressure signs continue and increase with the increase in charge weights?

    And, yes I know I probably shouldn’t have made 2 adjustments at the same time, load weights and bullet seating depth but I’ve got a match in two weeks and working out of town is putting a crimp in my development time. 😎

    Thanks
    Shane



  • Seating the bullet farther out will reduce pressure, unless you're into the lands.

    Pressure at low charges can often show up due to the case not getting a firm grip on the chamber walls and creating bolt thrust. You can usually tell the difference between a legit over-pressure round and a false positive like this, with a bit of experience.

    Pressure curves are a very complex thing. Its infinitely more complicated than people tend to think of it. The pressure curve (timing of the pressure wave) is just as important as the amount of pressure itself. If not even more so. Lower charge weights can show pressure due to undesirable pressure curves. That is why the goal is always 100% case fill, or as close to it as possible, with powders producing a desirable burn characteristic. The shape of the powder column at rest, the dimensions of the case neck/shoulder area, the bullet and its design/construction, all have a tremendous amount to do with this timing.

    Lots and lots of people run varget in Dashers with success, but I've found that in my 6 Dasher, 6BR, and 22BR... varget can produce some strange pressure spikes and weirdness. Weirdness that I've not seen since switching to H4350 in all of them. I find it to be the most desirable powder in these cartridges when running heavy bullets.



  • Plus one on the H4350, it’s incredibly stable, consistent and reliable in my 6br with the 105 hybrids.



  • @orkan
    Soooo.....don’t worry too much and carryon with my development then?

    Have thought about getting into the kids stash of 105 hybrids and trying them out too but,alas, that will have to wait probably till the snow flies....


 

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