Good video by Synchronizor:

“Many 870 owners wish to change the stock on their shotgun to better fit them, the application, or just their idea of “cool”. 870 stocks are not usually very complicated, and basic instructions are easy to find, but there are a lot of little lesser-known details that I often see causing confusion and problems for inexperienced owners, and there are a number of extra steps and tricks that aren’t repeated much outside the circles of gunsmiths and armorers, but are worth knowing about.

This video will cover the removal and installation of 870 stocks & grips in detail. It will explain the purpose of the stock bearing plate and when it should & shouldn’t be used, receiver sizes & stock interchangeability, and stock fastener lengths.”