This may not be the answer that you are looking to see. But----there ain't such an animal.
This horse has been whipped until he has given up the ghost.
Material things that are relatively low in price at one point are usually relatively cheap twenty years from now.
Guns, like other collectibles are to be bought out of passion and not as investment objects.
I would, sincerely, recommend that you look to land as an investment vehicle. Buy corners out in the country off a major highway and wait for MacDonalds, Super Six or other lease agents to approach you. You then can have a Dun and Bradstreet phone number printed on your business card.
The guns are for fun and pride of ownership. Not for investment.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.