I think the 257 Roberts is the cartridge that is becoming obsolete. I can't find any manufacturers chambering rifles in that round anymore and brass is very hard to come by and if you do find it, it's very expensive.
IIRC, the 250-3000 and 257 ROB were declared to be obsolete when the 243 WIN and 244 REM were introduced in 1955.
Performance wise, the 250-3000 and 257 ROB were as useful as dual-purpose cartridges as the 243 WIN and 244 REM under most hunting scenarios, but neither cartridge was a big seller in Remington or Winchester rifles, so Remington and Winchester introduced new cartridges with a lot of hype and velocity claims that were more sizzle than steak as a way to sell more rifles.
They did it again when they introduced the SAUMs, WSMs, and WSSMs, new cartridges with a lot of hype as a way to sell more rifles.
