cast bullets

I'm currently loading 150g jacketed bullets over 23.5g H4198 for @ 2100fps. I like it and it's accurate. Been casting for years so I made the leap and bought a .30 cal Lee 150g GC mould for my Mini which I plan to load over the same charge. I am wanting to load to maximum velocity, not target...

I am interested in anyone's experience in shooting cast bullets in a Marlin 1894 with the dreaded Micro-Groove barrel. I have one that I would like to use in CAS but one batch of cast bullets I tried at magnum velocities wouldn't group worth spit. I wondered if anyone has had much experience...