I’ll second the idea of fill speed, As lots of other things have been tried. I have a moulds for the 500 S&W from NOE and Lee. The Lee 460g fills great from the RCBS bottom pour. Now the NOE ( 4 cav, goes 375g, 450g, 525, 570g) won’t work at all with the RCBS – even the cav that is lighter than the Lee. Go to ladle pour, no worries !