Thanks for reaching out Eric! I want to make sure we're speaking on the same terms here. Now, to my understanding the Smith & Wesson Model 638 is only available chambered for 38 Special (full name: 38 S&W Special). The ammunition on this page is 38 S&W (also commonly known as 38 S&W Short). Since your revolver is chambered for 38 Special (as well as more powerful 38 Special +P), it cannot fire 38 S&W. The good news is 38 Special is a lot more popular. Your revolver can fire any of the ammo here: luckygunner.com/handgun/38-special-ammo
Perhaps you are accustomed to Remington's 38 Special 146gr LRN ammunition. I'm sorry we haven't got it in stock for you. I believe Black Hills 148gr wadcutter ammo will give you comparable (albeit not identical) recoil and ballistic performance: luckygunner.com/38-special-148-grain-hbwc-black-hills-ammunition-50-rounds
Please let me know if you have any other questions about 38 Special ammo!