Ackley imp. chambers are slighty bigger than the std. chamber. You have a gunsmith "ream out" the std. chamber to either lengthen it, take the body taper out, or both. Then you "fire form" your brassin the new "bigger" chamber. Once that's done, you can now reload "that" brass that now holds more powder to a higher velocity than the standard did.

I spent quite a few hours in Ackleys shop. He was quite a character, and i enjoyed every minute of my time there with him. I still have the 240 Wby. he built for me useing a 700 Rem. action and one of his bbls...

Drilling Man