A friend of mine reports that he was in a gunshop in France the other day and the owner was on the phone with a customer who wanted some .280 Remington Core Lokt ammunition with 165 grain bullets. Grains being an obscure measure in France, another customer asked what exactly a grain was. At which point an old timer stepped in to offer the following explanation: back when they first banned the use of buckshot for deer and boar in France and imposed the use of rifles, they settled on this purely administrative measure of a "grain" in order to show the equivalent power and effectiveness in terms that hunters understood. So a grain is a unit of power/effectiveness equivalent to that of a single pellet of buckshot. Thus a 165 grain bullet has the same effectiveness on an animal as 165 large pellets!
And there I was stupidly stuck with my idea that it's just an Imperial unit of mass equivalent to about 65 milligrammes or the nominal weight of a single grain of barley!