Side Saddle Disables Shell Carrier

[Zay_Shot]

A quite confusing issue I've been having with my dad's 870 express but basically when installing a Mesa tactical side saddle, the shell carrier stops working. Even the slightest amount of tightening of the side saddle and the shell carrier stops working.

Any idea of what could be going on/how to fix it?

Thanks!

[Scorpion8]

First off let's be sure of exactly what you have. Mesa Tactical doesn't list anything called the "side saddle", but they do have "SureShell saddle mounts". Is that it?