357 Mag Speeds...

[Rocky Raab]

Commercial cast bullets are probably harder than needed already. There are other factors involved in how badly - or if - a barrel accumulates lead fouling.

Just a few few of those other factors are barrel smoothness, forcing cone angle and smoothness, cylinder smoothness, chamber versus bore diameter, powder choice, primer choice, lube choice, and so on.

Some barrels foul horribly for no apparent reason and others don't at all. All you can do is try and experiment a bit.

I'd start with a good quality commercial bullet (most of the big names are primo quality), sized to one or two thousandths bigger than the groove-to-groove diameter and fired at moderate speeds by a fairly quick-burning powder and non-mag primers. Work up slowly in speed.

If you can fire ten shots without the group expanding like an umbrella, you're on the right track.