The Harvesters were too loose in my Extreme 50 and IMO that is mostly why i had the issues. Some of the 40cals were VERY heavy and/or long for caliber, so im sure that had an effect too. On one of the SML boards, fast 50x40 loads typically don't do well but the few that do are using the MMP at sub speeds from what i can tell.

BTW according to Blackhorn's website, 84gr weighed=120gr by volume. 50x40-200gr fps is in the 2150-2200fps range according to their data.