It has a lot to do with challenges of the mind, personal curiousity, and of course, male ego. The common calibers have never interested me like the "off" ones have. I currently have the capabilities to load over 60 different calibers. (keep in mind I have been doing this since I was 18 and that was 35 years ago!)
#1) challenges of the mind--Its harder and requires more thought to load for obsolete and/or uncommon calibers. Brass many times must be formed from another case-----reloading data is skimpy at best-----dies are usually expensive. Bullets many times must be cast or custom purchased-----------------(this don't make a lot of sense, does it?)
#2)personal curiousity--------When we all shoot a 38 Spl. or 30-30, we know in our minds what its going to be like, (you know, recoil, trajectory, etc.) therefore some peoples minds get bored quickly and some guys, like myself need a constant source of mental stimulation and excitement. In other words, I have adult ADD

Plus, my job as a forklift driver does not do a whole lot to keep me stimulated.

Thats OK though, because I 100% despise giving my mental challenges to a company that just wants to keep the shareholders happy.
#3) male ego-----Come on, we all want something special that the next guy has a hard time getting. Its called "individuality". "Hey Bill, guess what I just got? Its a .45-110-650!!!!" Why do you think the 1886 50-110 Express rifles bring so much money? Marlin and Winchester 219 Zippers bring 5 times what the same rifle brings in 30-30. Try to find a S&W .22 Jet. 25-35 and 38-55 SRC's in 1894's bring two to three times what a 30-30 SRC brings. These are all guns that were made in limited quantity. Its easily compared to paying for a '68 Camaro RS/SS with a big block motor and a M-22 trans with .456's------------Hope this explains it a bit.-----------------Sixgun
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