I lock one file in my vise. I then place bullet on the file and take a 2nd file and roll. Check the fit to the bore and if not tight enough slightly roll again. Once you get the feel it goes quick and you will know how much pressure and rolls back and forth it takes. I know my bore is between .400 and .401.

I already have way too many .45 smokeless guns. I wanted something different so I built the .40's.

If the current .45 McGowen barrels are tighter you will just need a Swinglock smooth sizing die to size the bullets. No knurl will be necessary as it adjusts to size for a perfect fit. You could get lucky and it takes the .451 Parker bullets without sizing also. Its hit or miss what the actual bore size will be.