The Streamlight TLR Low Profile Lighting System pressure switch will probably not work on a Pump action shotgun. Depending on what model shotgun you have the light listed at the link below may work.
https://www.opticsplanet.com/streamlight-tl-racker-remington-870.html
Follow up: I have in fact been able to install the Streamlight TPR Low Profile pressure switch on a Winchester 20 gauge SXP Pump shotgun, and the cord is in fact long enough, if you mount the light properly. You have to zip tie it to the fore stock, which is a bit mickey mouse, but it works.