Petersen 6mm Creedmoor brass weight

[jpjulian]

Since Lapua is currently unavailable I purchased a few hundred Petersen cases from Graf's. These are on average 16 grains heavier than Hornady brass. I believe Lapua is also heavier than Hornady but I don't know how much heavier. I know, and Petersen says, I need to adjust my loads to account for lesser interior volume of the Petersen case. My H4350 load is 40.9 grains with a Berger 108 in the Hornady case. My. thinking is 40.0 grains is a sufficient reduction. Comments?

 

[Benchrest Braxton]

i would think that as long as you weren’t on the edge of pressure with that load in your Hornady brass, a 1.0 grain reduction on the peterson would probably be fine.

 

[big5ifty]

You're loading anyway, just do two rounds of each in half grain increments from 40.9 to 38.9, and start shooting from the lowest over a chrony.

Change in case volume can easily account for a 100 fps velocity change with that powder charge.

 

[Jager]

Dropping 0.9 grains is probably enough. But unless you actually measure actual case capacities in the two brands of brass (using water), or put your old and new rounds over a chronograph, you can't really know.

FWIW, I found when going from Lapua to Peterson in .30 BR, it required a 0.3 gr reduction, using LT-30. That was with a 5.44 gr difference in average weight (Peterson heavier) and a 0.81 gr case capacity difference (Peterson holding less).

Also be mindful that once you've got that case_volume/powder_charge recalibrated for the new brass, you're very likely going to still have a tiny difference in load density compared to your old load and the pressure curve will be ever so slightly different. It's possible you might have to tweak further.

 

[drop_point]

You changed a component. Reduce load and work back up. There is no shortcut here.

 

[RegionRat]

Here is some advice for later on....
With your current question, which is where you have a previous baseline and are making a change in the case brand... this is an example of where loading/testing at the range is a huge advantage since you leave the range with answers instead of more follow up questions and trips.

For now, load very small tests before you mass produce anything.

It will answer your 40.0 versus 40.9 question without the risk of making too many of something you don't like. But, imagine having prepped and primed brass with you at the range, then you either pre-load a small ladder of charges to determine the effect of the brass change on your charge, and then make up as many of those on the spot for confidence before you commit to the recipe.

Sometimes with a change of brass, just matching the old speed isn't the whole story, sometimes it is. YMMV

 

[Beiruty]

 

[jpjulian]

I'm not a velocity freak and gave away my chrono a couple years ago. Damn thing lied to me anyway. All I care about is result on target. I probably should have waited a couple months to ask this question. I'm only home for a couple days, (funeral), and don't have my loading gear with me in Florida for the winter. Lucky to have a rifle with me down there. That said, I don't load to max pressure or max book velocities so if I did the typical load development of start low and work up, it would be a couple months before I'd realistically be able to continue with the work up. It appears, Beiruty, my proposed load of 40.0 would be well within safe parameters. Thanks for the graph. I think I'll take 50 rounds back to Florida and see how they fly. Worst case I'll only have a few to disassemble. Thanks for your advice.