Loading for my Rock Island 200 and my GP-100 I made 50 148 grain BB wadcutters with 4.5 grains bullseye. Data for the HBWC is abundant but rather sparse for the BB wadcutter. 4.5 is max in the Speer #10 manual with their wadcutter bullet. But there wadcutter is waxed pure lead and mine is range scrap and powder coated. Mine cast at .358 the went to .361 with the powder coat which will not chamber. I resized to .357 with a Lee die and a dap of Hornady Unique case lube. ( I lightly wipe my finger in it and just roll the bullet between my fingers )` They look great!
Then I loaded Lee 125 grain RNFP. These were cast and come out right below .357. Powder coated, lube and sized they gave little resistance in the sizer. I probably could have loaded them without sizing but I wanted some consistency. I loaded them with 4.6 grains of bullseye and 6.2 grains of CFE Pistol.
I figured the CFE Pistol would take up more room in the case. I was wrong. When I tossed the first charge with the measure at the same setting it tossed 6.8 so I had to turn it down rather than up. I didn't figure on the density being so much higher.
All this is to test the R.I M200, but I will shoot my GP-100 4 inch at the same time.