Problem #1: Factory ammo. Seriously. 1.5 inch groups with factory is GREAT, want better, reload. The example of the CZ527 in 223 is very relevant, mine groups about 2 inches at 100yds with PMC factory and I've shot groups as small as 3/8" for 5 at 200 with hand loads. The 45/70's I've worked with did significantly better with hand loads. The Marlin Guide Gun was a loser with factory ammo, at best 4 inch groups. Found a good load with H335. My Ruger #3 was very happy with 350gr bullets, didn't do as well with either 300 or 405's and preferred IMR4198. I've got a liner for a 12ga in 45/70 that shoots 12" groups, patterns is more like it, at 50yds with 300gr factory ammo and 2 inch groups with hand loads using 405s and H335. If you really don't want to reload for your 45/70 I suggest you get every brand and bullet weight available and do several trips to the range to find the "right stuff". With factory ammo prices you might be better off with just buying a basic reloading setup.