As I recall, trolls don't have bone spurs, they have keratin nodules that, like rhino horn, may have the consistency and durability of bone. And a weapon focus, going by both theme and game play, may enhance the combat capacity of an Awakened individual who has bonded to it - it makes them a bit faster, a bit more accurate, twitch the weapon at just the right time to hit harder. (And if you don't think that last is true, you've never learned anything about weapons, or even chopping wood.) Or it may channel mana directly into the target; that's in theme and game play as well, so it's up to you. What it doesn't do is 'add an aura' that does damage - well, not unless you have an elemental effect, and that gets wiggy, but still.
Also going by game play, we know that a weapon focus has to a) remain in the wielder's possession in order to work, and b) must aggressively and directly contact the opponent in order to function as a weapon focus. No arrows, no bullets, no shuriken - or rather, anything that is enchanted loses its active weapon focus capabilities just as soon as it loses contact with the person who has bonded to it, and does nothing unless it comes into physical (not auric) contact with a target.
As for me, if I hadn't already put together the dai-sho for Hawatari, which included enchanting the tanto, I would so enchant the pistol ...