If it were me I'd look to use something like a RCBS 45-500 FN without a GC and get that bullet coated if using a suppressor. Cast bullets have a lot less friction compared to jacketed bullets.
At 1000 fps it is not going to be loud, though a longer barrel would help if not using a suppressor. That particular bullet is designed to fit and function in the Marlin but importantly it takes up a lot of internal space, making reliable sub sonic loads easier to achieve.
