I personally think that a 300 grain bullet is too heavy for the cartridge. I you want to shoot a 300 gain bullet, buy the 45-70.
The Hornady XTP in 200 or 240 grain will be fine for deer. I prefer the extra speed of the 200 grain. I tried the 180 grain but fear that it is too fast and will come apart too quickly.
In a 240 JFP, Roze bullets offers one in a .431" diameter. Marlins have a larger bore in 44 mag. Usually reported at .431" or even .432"
