I am going to assume that you are talking a new Marlin 1895G. The newer ones have gone back to the Ballard cut rifling instead of the Microgrove as it tends to stabilize a wider range of bullets weights without the fouling assosciated with the older Microgrove rifles. My personal .45-70 will hold groups to around 2 inches at 100 yards reguardless of what i feed it. It is the Guidegun variation of the 1895 with an 18.5 inch ported barrel. The longer barrel version due increase velocity somewhat but do not seem to have that great an effect on accuracy.

The 300 gr ammo tends to blow up on very close shot except for the Federal with the Speer Unicore bullet. The 405 gr Remington is a better choice for hunting the thick stuff and will fully penatrate any blackie or deer but you ofcourse have quite an arc for those longer shots and a nice increase in recoil.

Personally all I ever shoot is the 300 gr Federal round or my own cast handloads.