As mentioned, the first thing to check, and most likely to cause mis-fires, is a loose strain screw. If you tighten the strain screw and still have mis-fires with factory ammunition then replace the mainspring with a new factory spring. If you still have a problem then replace the strain screw. The strain screw and a new spring will probably be lsee than $10.

Contrary to cmort's comment, if you find a mainspring that has a raised rib down the middle that is not a factory spring but, rather, a Wolf "Power-Rib" after-market spring. While there are several after-market parts providers I have never been convinced that any of their products are of any higher quality, or offer better performance that S&W factory parts.

First source of S&W parts is, surprise, S&W. Call Customer Service an 800-331-0852.

If you don't know where to locate the strain screw, remove the stocks if they cover the front of the grip frame. The strain screw is located toward the bottom of the front of the grip frame.