Depends on what you mean by "efficient," whichever powder gives the most velocity in a 22" will give the most velocity in the 18.5". But some faster powders might give less flash or blast. Some of the newer ones designed for military usage are low flash.
'Four legs good, two legs baaaad." ---------------------------------------------- "Jimmy, some of it's magic, Some of it's tragic, But I had a good life all the way." (Jimmy Buffett)
