I have Hammers loaded in 5 calibers and had the same thing happen in 2 of them, 25.06 and 280 Rem. I increased the crimp from 1/8 turn to 1/4 turn and it tightened the groups right up.
Nice rifle! I have a standard American as well in 243 win for a truck gun and it's one of my most accurate rifles also. Amazing such inexpensive rifles shoot so well.
Nice!
I have only shot coyotes with my 22-250 and 243, in several cases they were DRT. I just loaded some 90g Absolute Hammers for my 25-06 to use on hogs.
Try switching powder.
I have the same rifle, Ruger American 243 Win. When I started reloading for it there were no Ballistic tips available, so I bought 100g btsp Interlocks and 95g SST. I started with StaBall 6.5 and the 100g btsp and had the same issue as you, Hodgdon's max of 42.0g=3085 fps...