If you had a .17/50 BMG I don't think the barrel would last long

 

Correct. The 14.9 SOP uses the 20mm Vulcan as it's parent case, which is then shortened, necked down and then who knows what other steps are involved to make it a 14.9 SOP case. Then the bullets themselves have to be custom-manufactured buy the rifle's owner (or someone he has hired), as commercial 14.9mm SOP bullets don't exist.

The BATFE works in strange ways but, just like shotguns and large-bore (anything over .50 caliber) muzzle-loading rifles, other than the size of the bore, nothing about this rifle/cartridge falls outside the BATFE's definitions of a "sporting rifle". I believe that is the reason the BATFE gave this rifle/cartridge it's exemption. Everything about it is is custom, right up to the actual bullet flying out the end of the barrel. The exact same rifle chambered in it's 20mm Vulcan parent cartridge would never be BATFE Exempt as the ammo is already assembled by the military in all kinds of "destructive" variations. Now start making High Explosive or Incendiary bullets on your lathe for your 14.9 SOP (or your .30-06 for that matter) and the BATFE will be on you like stink on a turd!

It would have been easy to simply chambered this rifle in 14.5x114mm Russian, which has 175% the power or the .50 BMG, but due to the fact that "Destructive Device" ammo is already in existance for it, the BATFE would NEVER give it an exemption.