My go to load is a jacketed hollow point with 46.2 grains of IMR 4198. I wanted to load up some of the cast 325gr I was given. I normally get sub 2" loads from jacketed with a 2x scope at 100 yards off shooting sticks. I have spent the day loading 5 to 10 rounds then going out back and shooting them looking at groups and trying again. I have gone from 34 to 43 grains of IMR 4198 and adjusted my seating up and down but keeping it in the crimp grove. For some reason all my cast groups will create anywhere from a 6" to 12" vertical string. They group horizontally between 2" and 5". Its obvious vertical stringing which I have not had issue with before with jacketed. I pull some of my old jacketed ammo out and proceeded to shoot a very nice 3 shot group that went into 1.5". I have tried regular Winchester primers and also magnum CCI. The rifle is a ballard cut marlin cb. The bullets are Clip on WW and fairly hard, gas checked, and lubed with Red Rooster lube. Suzed .458 or .459. I need to get a micrometer and verify it this weekend.
I have not slugged my barrel but will this weekend when I can borrow a micrometer.
What Could be causing this?