If a cartridge is still in production, then it's not obsolete.
True, but I would consider some cartridges that are still in production to be obsolescent. In my mind, a cartridge is obsolete when neither guns nor ammunition are in production and obsolescent when ammunition is still produced, but guns are no longer available. Examples of obsolescent cartridges would include .32 Short Colt, .32 S&W, .38 S&W, .455 Webley, 7.62 Nagant, 8mm Gasser, and 7.63 Mauser. A cartridge can, of course be brought back from being obsolete or obsolescent if the production of guns and/or ammunition is resumed after a hiatus. Examples of such are many old west cartridges such as .44 Russian, .45 Schofield, .38-40, and .32-20 which have been revived due to interest in Cowboy Action Shooting.
Of course, .38 Special has never had a hiatus in production of guns or ammunition (the S&W Model 10 holds the title of longest continuous production handgun) so it is neither obsolete nor obsolescent.