My 1895 Marlin (1979 strait grip vintage) has a 12 groove MG barrel (some 16 groove barrels were also released) and shoots a half-lino half-WW RCBS 45-405 FNGC bullet ~ .75" @ 100yds. It does the same with the 400grn Speer Hot-Cor jacketed bullets ~1850 - 1900 fps. I love that rifle :-) Wears a Decelerator and Leupold 3x now. Took a nice 5x5 high country bull with it two seasons ago.
Jeff

my marlin 45-70 shoots great. a guy i know had it for sale, and offered me and my friend to take a shot. having never fired it before, i accepted. bullseye, offhand, iron sights, at 50yds. i am now the owner of it, and other thatn my mauser byf42, and my model 70 (still in the shop ) it is my favorite rifle. i also ended up buying a model 1200 12 ga from him. beautiful shotgun. he'd make a good salesman, huh ? seriously, the rifle is very accurate up to 100 yds with winchester factory (300 gr) ammo. have not tried at a distance beyond that with it, because that is the longest range at our gun club. that is a true story, believe it or not.

I shot a bunch of sub MOA groups last weekend with mine. Almost half MOA for some. http://www.cylindersmith.com/45-70LINK.html