Ammunition obsolescence is a squirrley concept when it comes to firearms.
I look at it as being a two stage process...
1. Makers quite making guns chambered in the round.
2. The big ammo makers stop providing loaded ammunition.
Many rounds are going into and out of obsolescence all the time as interests and tastes change.
Cowboy Action Shooting and the other "old west style" games like blackpowder cartridge rifle silhouette are probably responsible for the resurrection of more obsolete rounds than any other activity.
A couple good examples of rounds that fit in category 1 above are the .250 Savage and the .300 Savage.
Both were exceptionally popular in their day, and remain so, but have been overtaken by other rounds such as the .257 Roberts and the .308 Winchester. Every once in awhile someone will make a run of rifles for one or the other, and both still sell a lot of ammo, but they are slowly churning to a stop.
I figure in another 15 to 20 years, though, someone is going to "rediscover" just what an incredibly well balanced round the .250 is and it will stage a nice comeback.
I love my .300 Savages, but if I were choosing a single cartridge for eastern hunting, varmints all the way to black bear, I'd seriously consider the .250 in a bolt action.
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