Ok, normally when these threads come up, my first twitching-eye, knee-jerk reaction is to tell the poster that guns aren't a good investment (they aren't) and that you're better off with even just a plain old index fund.
HOWEVER, since you stated you've already got those bases covered, and you're just looking to invest purely in guns with what amounts to fun money, there are a few that you won't lose money on, and may even appreciate (if you don't pay too much attention to inflation.)
I guess I'd look into the following categories:
Stuff people can't get.
Oddball stuff they don't make any more.
Stuff with a high nostalgia factor.
Stuff with a high price tag that not many can afford.
Look into Machine guns first. A few months back, someone posted a link to a website that had (very, very) rough graphs showing what particular models were being bought and sold for. I can't find the link at the moment, but you might want to poke around in the archives.
I'd look at Colt DA revolvers, as well as Colt SAA's from the 1980's or earlier. Winchester lever-actions should be a consideration as well, but it's a bit of a gamble (like any of this.)
High-end clay shotguns will hold their value, but you should research whether or not they'll go up in value. Ditto for safari double-rifles.
A lot of WWII stuff tends to appreciate in value, but you need to research it pretty heavily and go into it with your eyes open. The market for war-era 1911's, Garands, Liberators, and other guns is pretty well staked out, and there are a lot of people who've been buying, selling, and trading in that market for years who obsess over date stamps, serial numbers and proof marks.
Other than that, look into weird stuff that's no longer made; the Dardick pistol (as well as ammunition) or the Gyrojet. Check into other rare stuff that most people haven't even heard of.
In the end, it's all going to be a gamble, and unlikely to appreciate at the same level as even an index fund. Also, this post is just my opinion, posted on the internet. If you follow the information it contains and end up losing a ton of money as a result, it's your fault, not mine.