I have a number that were gifts that I could never sell or trade. The oldest is my 1st .22 rifle - J.C. Higgens single shot bolt action dating back to 1952.
Of those that I've bought, the real keeper has been a Ruger Mark I bull barrel from about 1967. I'm not sure why I won't sell it. I've got similar guns that are nicer and more accurate (S&W Model 41, a Ruger Mark I with the long barrel - of about the same vintage, and so on), but every time I think of trading it off for something I rethink and don't. Aside from the fact that I wouldn't get very much for it (it isn't pitted, but the blueing is very, very worn), I think back on where it, and I, have been. It is accurate enough to feed me in the woods (again) and it just works.
My wife gave me a nice stainless Mark II bull barrel, which of course I will never trade, but when I grab one up to go for a walk in the woods or squirrel hunting, it is usually my old friend.
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Jim Fox