I'm trying to word this so it doesn't get moved in the caliber corner, so in the AR platform, what on your opinions on the 5.45x39 vs the 5.56x45? I suspect the majority will say 5.56 because it's "what they already have." Obviously each has it's own advantages, but can you see yourself picking up a 5.45 upper IF you had the cash as a possible cheaper training option? If cash isn't an issue, then would you ever consider the 5.45? As far as I can tell, the main points are as follows(please feel free to add any pros or cons in your comments): Pros for 5.45 are: Cheap Comparable ballistics to 5.56 Easy conversion to 5.56 Cheap Cheap Cons for 5.45 are: Not many U.S. applications Not a lot of different bullet weights and combinations Most ammo out there is corrosive AND Pros for 5.56 are: You can fill in the blanks yourself Cons for 5.56 are: Getting outrageously expensive So simply for the argument that it's less than half the price of 5.56, is it a viable training option? Why or why not?