Hard bullets with a beveled base are the worst at leading. Most likely you have slightly oversized chamber throats and instead of the base of the bullet expanding (obturating) to fill the throat and seal the gases behind it, the gases are blowing around the base (with help from the bevel) and melting a portion of the base and sides onto your barrel throat and bore. Not being a caster yourself, you will need to see if the seller of your bullets offers them in larger sizes, .430 or .431. If you were a caster, a square based bullet sized .001 or .002 larger and cast of softer alloy would surely cure the problem. With the more recent lots of 2400, 20gr should be more than enough. That's good for more like 1300fps, not 1200. 21 to 21.5gr of 2400 is absolute max. Increasing the charge is not the answer.