The mobility in Destiny makes it so that range stat adjusting simply will not fix the problem. It's time to think outside the box a little bit. The real fix is to make no shotguns be able to one shot at all (with the exception of Chaperone precision shot, keeping the exotic feel and not being exceptionally overpowered) and thus require a follow up melee or second shot. That alone will set the perfect shotgun range, require the users to be in that range, and still be very easy to get the kill if you get into that range. Not only will that set the perfect range for a near instant kill, like a shotgun should to reward getting that close, but it would also make all shotguns viable for crucible. You can choose to use a lower impact shotgun and shoot a bit faster for the follow up shot or use a high impact shotgun to guarantee the kill on the melee follow up that you're already closing in for. That would also then allow sidearms to be good choices for just past melee range, fusion rifles to be the best choice for medium range, followed by snipers past the medium range. It's literally the perfect solution.