I've been testing two new 20 cal magnum I plan to use for coyote hunting
I started off with 20x47L and RL-17.
The loads maxed out around 42.5 grains at a little over 4200fps.
Most accurate loads were at 41.0 and 41.5 at 4125 to 4150fps.
I was shooting 38gr unmussig bullets.
Next I shot some fire form loads in the 20 250 improved with 40 grains of 748 and 39 grain Sierra's.
Pressure was a bit high on these but not excessive. The real surprise was the velocity! These were humming along at 4500fps. Unfortunately most of the bullets didn't make it to the target.
I forgot to mention that both barrels are 1:12 twist.
I'm thinking I need to try the 748 in the Lapua as well. I thought it was going to be a bit fast for these case capacities and bullet weights but it may be just the ticket and I have about 16 pounds of it. More than enough to burn up both barrels.
I purchased both of these while waiting on my 20 chupacobra that is being built. It's based on a 22 250 improved with the addition of the shoulder moved forward as well. After today's testing I'm thinking I may be able to get about 4700fps with the Chupacobra and the Unmussig bullets.
W748 may be a great powder for this capacity and bullet weight.
39g Sierra's will not withstand 4500fps
Back off fire form loads to 38 or 39g.