In my area of California while the crazy element is still present it generally means they mod their weapons in particularly volatile or weird ways, maybe they have an obsession with a particular era of weapons as well. Honestly it's a pretty informal term where the only useful measurement is that most folks who could be described as gun nuts probably press their own ammo.

So it can be a neutral term depending on region at least, either that or my Redneck is showing. Frankly speaking though I have seen folks (usually yuppies) throw fits because folks have either rare or "scary" looking firearms, apparently there was an incident at a gun range relatively near me where some stupid bitch called the cops cause a dude pulled out a Browning Automatic Rifle.