RE: ML Theory ?
My uneducated guess is stability.
Rifle's have twists around 1:10"....ML...anywhere from 1:28" to 1:70" I think if you were to try toload such a small diameter bullet down such a fast twist barrel...you would severly mangle the bullet, thus getting eratic shot to shot preformance.
Plus with a smaller diameter barrel, you would lose the area needed to hold enough powder for adaquate power.
I could be, and probably am, way off...but that is what popped into my mind.