It's possible that he has a poorly made powder measure, or perhaps a DIY and only thinks that's 60 grains. The barrel on the only derringer that they sell the might hold that much is the Philadelphia Derringer with it's .45 caliber barrel and 3.0625" barrel. I'd venture to guess however, that either he just fills it up and fires blanks, OR his patched round ball is barely below the edge of the muzzle, so he's not getting anything other than a good BANG out of it. I don't think it would have much MV as I suspect it clears the muzzle too soon and thus doesn't accelerate much. Safe? Probably would be more dangerous if he loaded it with 45 grains and seated the bullet deeper..., but I guarantee it's exceeding the loading manual's recommended max load.
LD