I did some load dev yesterday.
A couple of takeaways.
We had to size it down in three steps. Because of the necking down process, the COAL was all over the map, making fire forming a must. Once the brass was FF, this rifle was very accurate.
It takes three more grains of 570(69) to match the velocity of a 6UM.
When we average out the speeds of this 20" 6-7PRC barrel and three other 20" 6UM 20" barrels, the 6UM and the 6-7PRC are pretty much ballistic twins. However, a few more 6-7PRC 20" barrels would give us better data.
570
Three shot average
70g 3370 HOT!
69.5g 3315 Hot-ish
69g 3290 shootable but on the edge
68g 3230 easy peasy
Retumbo
68g 3396 HOT!
67g 3320 HOT!
66.5g 3315 HOT-ish
66g 3260 warm
65g 2195 easy peasy
565
64g 3161 HOT!
63g 3142 HOT!