Re: 6mm/220 swift?
I own and shoot an original 6mm Lee Navy. But of course with that straight pull action and pressure limits it not a barn burner.
If you are looking to making a 6mm-220 Swift Improved, you may think about building a 6mm Lee Navy Improved. The Lee Navy has a very long neck compared to the 220 Swift. You can buy 6mm Lee Navy brass from Buffalo Arms. Pricey, but it can be bought. It is made from 25-06 brass sized down and the head trimmed to Lee Navy specifications.
That would be the case to base an improved version on. You could put a false shoulder on the neck and gain a quarter inch of case capacity blowing it out. Would be a hell a lot of trouble however compared to other options...but it would be unique.
Beverage of Choice = Jeremiah Weed
Weapon of Choice = 30 mike mike gatlin gun