Originally Posted by Guv
How does a 45Colt head space if you remove material from the back of the cylinder? What maintains the distance between the primer and the firing pin?
You don't remove all the material.
You mill out material between the extractor star and the middle of the chambers. The area of the cylinder which was not milled still supports the rim of rimmed cases. The are that was milled supports the moon clips that support the cases. Thus headspace (the distance between the breech/firing pin and cartridge/primer) is maintained for each type of cartridge either way.