I've had great results on deer with the 250 gr. factory load and with the 220 gr. Hornady. I shot an average size doe head on in the chest at the base of the neck. The 250 gr bullet penetrated full length and exited near the tail.
For cast bullet loads, I use fire-formed 30-30 brass. It is thinner than .375 Win. brass and works well with slightly over-size cast bullets. Mine works well with .379 Laser-cast bullets. They grip the micro-groove rifling better than .377 cast bullets.
I have what is probably a life time stash of factory ammo and 220 gr. Hornady bullets. I still like to experiment with other bullets. I've found that the 260 grain .375 Nosler partition can have the nose filed off a bit, to get a flat point. It is accurate if you get a consistent length.
I also use 38-55 brass from Starline. It is thicker than fire-formed 30-30 but thinner than .375 brass. It works with cast bullets. I've found that the chamber on my Marlin is long enough that 38-55 brass doesn't need to be trimmed.
