Parker Hale 2-bander question

Hi everyone,
I think I already know the answer to this question, but thought I'd also get some opinions from you all.

I'm trying to tweak the 100 yard accuracy with my Parker Hale 2-bander, and so I'm thinking of moving to a heavier bullet. This would mean going from my current Lyman 575213 minie (510 gr minie, which shoots pretty well in the gun already) to a heavier Parker Hale style Minie (560 gr). I'm hoping the extra mass in the PH style minie and a bit more powder will help tighten the groups a bit more, particularly at 100 yds.

I have been sitting on the Parker Hale mold that came with the gun and tried casting with it today for the first time. The minies out of this Parker Hale mold are only about .573 in diameter, whereas ordinarily I size the Lyman 575213 rounds to .576".

Do you think the Parker Hale minie going to be too undersized for what I want it to do? Should I just get the Lyman 575213PH mold and size to .576 for a tighter fit in the bore?

For what it's worth, the Parker Hale mold threw bullets today in the 547-550 grain range, whereas the Lyman 575213PH is listed as a 566 grain minie.

I did manage to get to the range tonight before dark and shot four groups of five at 100 yds (well, actually 96 yards since our frames got moved in a bit). Groups were okay, nothing to brag about, but I was shooting off cheap bags rather than my good rest. Started with 45 gr of 3F, groups were a tad high, might drop the powder a bit and move back to a portable bench at a genuine 100 yds. I'd say the groups averaged about 5" without much effort on my part.

What I'm hoping is that heavier minies will tighten the groups at 100 yds, especially given the relatively fast 1:48 twist in the P58 rifling. other thoughts?

So even though I'm pretty sure I'm going to need the Lyman mold, I just wanted to see what folks here thought about the sizing issue before spending the cash.

thanks everyone,
Jim

ps. I also don't want to grind down the base plug on my 575213 mold, would rather keep that intact in case I want to go back to it later.