So I've got a new-to-me American Knight Rifle, .50 cal, in decent shape for the shape it's in. I've got a few projectile options to try first - Powerbelts, Cheap Shot Sabots, and sabots with .45 cal Hornady FTX 250 gr. bullets installed. BUT I also cast my own boolits (technical term) for rifle and .35 cal pistol/rifle. I don't have any .50 cal molds, but is there any reason that I can't cast my own conicals, lube them up and fire away for a less expensive projectile option? I've always read that pure lead is needed for ML projectiles, which I have, as well as other alloy options. Thoughts? Experiences? I know cast boolits are great on game, but will I get the same potential degree of precision with cast projectiles? I keep my current cast projectiles to +-.2 gr. weight deviation.

Yes sir

No reason at all. Cast away, and go fill the freezer.

It really depends on your rifle. Usually todays modern inlines have twist rates that do not favor the cast conicals.My inline will not shoot my cast maxiballs, but does great with Powerbelts.Where as my TC Hawkin will shoot the maxiballs very well. Before investing in a mold, I'd buy a box of maxiballs if that is what you are looking at and try them.

The Knight should shoot the Hornady Great Plains bullet good, so cast something similar.

What diameter mold and boolit weight should I be looking at for casting for a .50 cal ML with 1-28" twist?

375-425gr.

So would the Lee R.E.A.L. 320 gr. .50 cal be a good place to start?

You can try it, but I think it's too short for a 1-28 to be ideal.

385gr to 460gr is probably better. 440gr Bullshops usually shoot really well in a 1-28.