I don't think you are expecting too much. I once had a nice little Marlin 336 in .35 Remington. I reworked it to improve accuracy using a "how-to" article in some magazine or other-can't remember which one! But in addition to improving the trigger pull, one of the modifications was to remove the barrel/magazine tube band from the muzzle end of the barrel, and cutthe magazine tube fromfull-length to 2/3 length. In addition, Irelieved the receiver end of the magazine tube where it contacts the receiver, so that the magazine tube doesn't push any harder on the receiver when the barrel is cold than it does after the barrel begins to heat up and expand, moving the magazine tube further away from the front of the receiver and thus placing less upward pressure on the barrel via the magazine tube hanger. When this happens, you can have vertical stringing. There may have been some other changes too, but if so, I just don't remember them.

When I was done, this rifle would shoot MOA with both 180-grain Speer handloaded flatnose bullets, and the RCBS cast 200-grain .358" flatnose gascheck bullets as well.(This 200-grain cast gascheck bullet was also a great performer in the .357" Magnum with 9 grains of 2400!)

My .45/70 is a Ruger No. 1 with 1/22" twist. I have never gotten it to shoot well with 300-grain bullets-IMO, they are too light. BUT, it will shoot well with the Lyman-Gould cast .457122HP, which cast from wheelweights comes out at 342 grains from my mould. This rifle also does well with the 400-405 weight bullets from Barnes, Speer, and Remingon. I too had large groups and fliers with 300-grain bullets - I don't like 300-grain bullets in .45-caliber rifles! Their sectional density is too low for use on anything bigger than deer, anyway!

I'll bet your rifle will shoot heavier bullets better too.