First I want to clarify that in some US jurisdictions, black powder weapons are treated the same as centerfire or rimfires.. basically banned.. unconstitutional but don't be surprised if the business will not ship to you. In most places they are considered non firearms, and I am in fact building a collection of Scottish clansman flintlocks, and will eventually be buying a firing replica brass barreled blunderbuss and also a Brown Bess musket, for display in my arms room/dojo. I will shoot them once in a while, for the nostalga. I would also enjoy a blackpowder .44 revolver carbine, or 12 GA of same type, or 4 barrelled old west shotgun, but I don't think anyone manufactures them right now.
One currently available gun only? Well I'll give one of each type, since it's unthinkable to not have one each rifle, shotgun, and handgun:
Rifle.. hands down my tricked out HK91A3 .308
Shotgun: HK or Benelli semi auto 12 GA
Handgun.. this is easy, one of my customized Browning Hi Power 9mms
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If the issue was say pure survival after a nuclear war or alien invasion :ak47: (grin) and I could only have one it would be the HK91A3. At least I would be looking good with a heavy hitting rifle, until the aliens bagged and BBQ'd me or I found out their secret weakness for fried chicken, which would explode their cellular structure :uhyeah: