I'm not sure there would be enough meat in a blank to machine the groove required in a vulcan barrel. If there was, You'd have to be pretty gentle when closing the gun to keep from deforming it.
I guess the bore of the breech block could be enlarged to allow the full diameter of the blank to be used right up to the breech. Come to think of it, the part of the barrel engaging with the pin could be achieved on one side only, giving extra strength.
What diameter do the lw / bsa etc blanks come in?