Tony,
I agree, it is difficult for me to believe such a large varience between my loads and yours/wlktheduk's. I'll pull a bullet and double check. As for the plated bullet being that much faster, I'm not sure. I'd say its a definite "mabe". I know that lead bullets are faster for any given powder charge b/c of less resistance going down the bore, and plated bullets are between lead and jacketed in that respect.
I'll do some follow-up tests and post what I find.