I personally would take the cylinder stop out in order to inspect and clean the entire interior of the gun thoroughly. (See the video below; I use Q tips instead of the brush, and I'd probly wipe it out with Hoppes 9 first) Then, if all looks normal, I'd reassemble using a Wolff regular strength replacement cylinder stop spring.

If you have any mechanical skill at all, disassembling and reassembling a S&W is easy.