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.