I thought that a "Modern Sporting Rifle" was any firearm that used self contained metallic cartridges! I consider my Sharps 1874 in 45-70 a modern sporting rifle (at least compared to my best friends 50 cal. Flintlock!)

As to the dilemma of you Black Rifle, spare parts are a must, I acquire them all the time, usually already assembled in a working gun configuration! There is no reason not to have a spare bolt carrier group. I never ask my armorer friend, but I don't think most agencies rebuild fully functional weapons! But they seem to buy lots of spares!

Ivan