Jim. I have an early "new model" Marlin 95 with the Micro-groove barrel and painful curved buttplate. I get from 2 to 2.5 inch groups with a peep sight most of the time,when it isn't kicking the slats out from under me. (I load some pretty warm loads for hunting)
All I shoot is cast lead in this rifle. I have had the best luck with gas checked bullets, but at the velocities involved with "cowboy" shooting, plain based bullets may work as well. In fact, don't "cowboy shoot" rules specify plain base?
Anyway, use a cast bullet that is at least .002 inch larger than your bore diameter. My bore is .457 inch in diameter. My bullets cast .460. inch. I could probably hand lube them and shoot them as cast, but I have been sizing them down to .459 inch. Use a relatively hard alloy as this way there is less chance of the bullet stripping. Also use a hard lube, such as LBT Blue, or Lyman's Orange Magic. Alox 50-50 will work but it is messy and it stinks like a skunk. Gets runny on hot summer days too.
The only plain base bullet I use is Lyman's 330 gr. hollow point, the Gould bullet. Gould was the president of the NRA back in the late 1800's and possibly into the early 1900's. He designed an express bullet (Lyman #457122) for the 45-70, which if properly cast shoots well in Micro-groove Marlins. If you can find a copy, get Paul Matthews FORTY YEARS WITH THE 45-70. He goes into loading for micro-groove Marlins, and gives the load I use with the Gould bullet. Actually that's where I got it. LOL The alloy I use is 10 pounds of cleaned wheel weights, and 1 pound of linotype. Makes a good bullet as is,(12BHN) and can be heat-treated to be much harder, if necessary. (20-24 BHN depending on WW composition)
Hope this helps you out.