Howdy,
Ditto what they said. I just spent a frustrating summer trying to dial in a Marlin 1894 in .41 Magnum. After delivering shotgun patterns at 25 yards, Marlin said to send it in and they'd look at it. After I got it back, there was no difference. Tim Looney, head Marlin Tech, alluded to the fact that their barrels ran slightly over standard groove diameter, and I should shoot factory ammo as it was "hotter" that what I was using. He went on to infer that I didn't know how to assemble a proper handload and Marlin wouldn't be responsible for my poor results, and that they'd buy it back if I wanted to ship it back with the receipt.
That conversation pissed me off. I decided that, since they were going to take it back, I'd try some cartridges loaded to the utter maximum. If it broke, so what? That did the trick. The rifle shoots one-hole groups at fifty yards using the highest published charge of 2400 I could find.
Try using bullets at least .431" or even .432" in diameter. If using jacketed bullets, rock them hard! Use 2400 or H110 and swell them into the rifling with a maximum charge. It worked for me.
Good luck,
Bitterroot Bob