Edit: A lot of people say, that GWM needs a melee weapon attack, but they miss Jesses point: While GWM requires a melee attack with a heavy weapon, Sharpshooters only criteria is an attack with a ranged weapon (not a ranged weapon attack). Jesse bases his claim on the fact, that a crossbow is still a ranged weapon, even if used as an improvised weapon for melee combat. That’s why it deals 1d4(!)+20 damage. (It works with any ranged, heavy weapon btw., so Longbow qualifies too.) Of course Jesse is playing the devils advocate here and of course, no somewhat sane Walter will allow this in any campaign ever, as it’s obviously not the intention behind these feats. But you could read it that way and that’s Jesses (paperthin) point. Besides: he finds the image of a barbarian running around recklessly smashing a crossbow over everyone’s head to just be hilarious.
I’m one to rule with intent, and would rule against using it, but at the same time, it does say
" Before you make an attack with a ranged weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If that attack hits, you add +10 to the attack’s damage."
Mhm yeah, but if you attack with a ranged weapon in melee, its no longer an attack with a ranged weapon as you use it as an improvised weapon instead.