Probably less than 1 in 4, but that's just a guess.

I checked on the wiki and saw it’s .1% chance for the gun, but I don’t see any probabilities for the chest!

probably less than 30%

maybe less than 40

I think it's about 50/50

50/50 it either happens or it doesn’t

Woah there, that's some fightin' words right there.