How far does the butt of the rifle need to move to change the your point of aim or impact 1 inch? Its an incredibly small amount. The amount of movement is dependent on the distance between the front support of the rifle and the rear support. As this distance gets shorter the allowable movement at the rear of the rifle becomes less and less. For a rifle that is 36" from the rear bag to the Bipod the rifle only needs to sink 0.01" to move your point of impact 1".
How we use the rear bag is a critical portion of the sighing system. This is especially important during the recoil management process as the rifle starts to move before the bullet exists the barrel. Having a good, solid rear bag is very important. Having a solid squeeze on that rear bag and maintaining this pressure through the shot process is critical to have consistent and repeatable shots down range.
The Diagram
This is an image of the movement. Even a 20MOA change is less then 1/2" at the butt stock.
The Math
Here is a quick look a the math and trigonometry that goes into the calculation
The Table
Here is a table for the rear bag movement required to move 1" at 100yards based on the distance between the supports. If you wanted a 1/2" at 100yds divide these in half. If you wanted 1/4" at 100yds divide these by four. To shoot a 1/4" group at 100 yards the rear of your rifle needs to move less than 0.0025" (literally the thickness of a human hair (blond))
The Real Question
If we and our rifle are shooting 1" groups but we want 1/2" groups; Should we really think its the ammunition that is not working??? ... ... Or is our rear bag, and how we are using it, not good enough? #Fundamentals