07-09-2024, 02:15 AM
Y'all are running into the same issues that have confronted me. First, note that Hornady's data is with their brass, which differs from Starline capacity. But (not being patient enough to get into QL) I've developed a workaround for me in Grt, that works, although truth be told it is not perfect and won't cover all the bases.
First caveat, this is info only, and use at your own risk. It works for me in a limited way, but beats not having anything to go on as well. YMMV.
Even with Grendel there was talk somewhere that these short powder-column cartridges, with small diameter bullets, needs some tweaking to even come close. One thing is that (theory) the primer firing can unseat the bullet enough to make the chamber effectively more "room" than just measured cc capacity, until the bullet obturates and seals the gas behind it. Another is that the Sebert factor differs for these cartridges from the default of 0.50... due supposedly to some of the powder "following" the bullet farther down the barrel more than the more standard, 308-style cartridges. I can't prove or disprove these, but I did put them together for my workaround...
1. For 6Arc and 6.5 grendel, I change the Sebert to 0.45, a factor that I played with a bit to find something that works pretty well. ie, more powder follows down-barrel when Sebert is below 0.5.
2. I use the hornady data (or hodgdon for that matter), and develop a "calibrated" round for a Hornady bullet, using bullet, oal, and powder charge, then adjust the case capacity until I get the Hornady book value. From that cal value I can find 1 or 2-off load data that pretty much tracks with my results.
For bolt action I use the 62ksi limits and Hornady bolt action data. Hodgdon sticks to 52 ksi for its data.
So if you do this, you would start with the 87 gr Vmax as your bullet, at the book oal of 2.140 (iirc, going from memory) and stock 6Arc case data, then add the powder and charge weight, 18" barrel, 52ksi limit, then change the case capacity to get a "calibrated" load sheet. Save this as "18-AR-87Vmax-powderxx-cal.grt" (example load sheet name), then you can do 1-offs from it, sometimes 2-off deltas. like swap in the 87 gr Amax, and move its seating depth to your load to get a feel for it, and your barrel length. Save your 1-offs under a different sheet name. I store each rifle, each barrel length, basically in its own sub folder. Organizing helps you retrieve load data.
I don't know in Grt how to "calibrate" powder burn rates or energy which can vary up to 10% lot to lot, so this case cc delta is my workaround (I've seen 1-2% but never 10, but there you go). You won't be able to always tell exactly what % load you're at, maybe in general, but from your loading bench you can develop a feel for it.
Anyway it has worked for me as a usable guide. I'm sure there are better ways to do it, but this is one I've found for me.
Edit to add: once you have chrony data you can tweak your specific load sheet to match your actuals, I usually use case capacity again, along with the other real world data from that cartridge load. I then save it and add "chron" to the file name.
First caveat, this is info only, and use at your own risk. It works for me in a limited way, but beats not having anything to go on as well. YMMV.
Even with Grendel there was talk somewhere that these short powder-column cartridges, with small diameter bullets, needs some tweaking to even come close. One thing is that (theory) the primer firing can unseat the bullet enough to make the chamber effectively more "room" than just measured cc capacity, until the bullet obturates and seals the gas behind it. Another is that the Sebert factor differs for these cartridges from the default of 0.50... due supposedly to some of the powder "following" the bullet farther down the barrel more than the more standard, 308-style cartridges. I can't prove or disprove these, but I did put them together for my workaround...
1. For 6Arc and 6.5 grendel, I change the Sebert to 0.45, a factor that I played with a bit to find something that works pretty well. ie, more powder follows down-barrel when Sebert is below 0.5.
2. I use the hornady data (or hodgdon for that matter), and develop a "calibrated" round for a Hornady bullet, using bullet, oal, and powder charge, then adjust the case capacity until I get the Hornady book value. From that cal value I can find 1 or 2-off load data that pretty much tracks with my results.
For bolt action I use the 62ksi limits and Hornady bolt action data. Hodgdon sticks to 52 ksi for its data.
So if you do this, you would start with the 87 gr Vmax as your bullet, at the book oal of 2.140 (iirc, going from memory) and stock 6Arc case data, then add the powder and charge weight, 18" barrel, 52ksi limit, then change the case capacity to get a "calibrated" load sheet. Save this as "18-AR-87Vmax-powderxx-cal.grt" (example load sheet name), then you can do 1-offs from it, sometimes 2-off deltas. like swap in the 87 gr Amax, and move its seating depth to your load to get a feel for it, and your barrel length. Save your 1-offs under a different sheet name. I store each rifle, each barrel length, basically in its own sub folder. Organizing helps you retrieve load data.
I don't know in Grt how to "calibrate" powder burn rates or energy which can vary up to 10% lot to lot, so this case cc delta is my workaround (I've seen 1-2% but never 10, but there you go). You won't be able to always tell exactly what % load you're at, maybe in general, but from your loading bench you can develop a feel for it.
Anyway it has worked for me as a usable guide. I'm sure there are better ways to do it, but this is one I've found for me.
Edit to add: once you have chrony data you can tweak your specific load sheet to match your actuals, I usually use case capacity again, along with the other real world data from that cartridge load. I then save it and add "chron" to the file name.