Cylinder difference

[HogLegBill]

How can you tell the difference between a 45 colt and the 45 acp cylinder for a blackhawk? Is there a ring or grove in the back of the acp?

 

[rugerguy]

you have to look "inside" the chambers, you will see the 'ACP' chambers 'ring' up close ( short) and the long Colt ' ring' is further down, closer to the front.......besides, the 'ACP' will only go in this far, as it head spaces on mouth ( rim) of the case.........
old models may have a firing pin groove around the breach end of the cylinder.......