Bag rider to improve accuracy

[Lemonaid]

If your stocks has a flat edge then a bag rider isn't needed. Just put your rear bag under the flat section.

I'm using a Magpul moe stock which has about 3/4 inch flat spot at the bottom end so under recoil it will move/slide on the 45 degree angled part, much like in the Mark and Sam vid. causing the muzzle to move up.
A similar if to a less degree situation with the RAP stock.