I know in rural New York state at least (note this isn't New York City, I only point it out because I know way too many people that aren't from around here that get seriously confused that the state is much bigger then the city) it's legal to get a longarm or a shotgun at 18 if you also have passed a firearm safety class which is usually accompanied by getting a hunting license. And assuming you own the gun legally you can carry them around in your car as long as they're unloaded and the ammo's somewhere else (usually the trunk) and since it's usually assumed you're out hunting....something most cops won't say anything about the guns unless there was recently a shooting or they're asking if you got anything while you were out. Walmarts and some sporting goods stores around here will sell basically any kind of rifle or shotgun and the ammo needed to shoot them though I can't say if there's a waiting period since I got all my guns second hand as gifts and such. Fully automatic weapons aren't sold however and are generally illegal.

Pistols are harder to get and more illegal to own requiring permits and background checks and waiting periods since they're viewed more as weapons for killing people then for use as hunting implements. The ammo's easier to come by though.

Bows and crossbows are legal to own and shoot though you'll likely get odd looks carrying them and some people won't recognize a traditional bow for what it is as they've only seen the more advanced compound bows with the pulleys and laser range finders and such.

As far as I know most of the rural US has similar outlooks on guns if not the actual laws. Mind a lot of that is more anecdotal evidence then actual readings of the laws involved. I also know that when you get closer to bigger towns and cities the local police tend to get more attentive as to what's legal for you to be toting around then out in the country.

Do all fae have to have "The Catch is Cold Iron" in the Dresdenverse? I wonder this because redcaps are most commonly described as wearing iron boots and wielding iron pikes, axes and hooks. I don't really know what to replace the catch with though something to do with the dyeing of their hats in fresh blood to maintain their power might be good.

They also are described as "Inescapable". Once a redcap begins to hunt you there isn't a way to avoid being found. Maybe model that as some kind of tracking power or even give them Inhuman Speed?