Mike,

The Caldwell Lead Sled is excellent for taking the sting out of load testing a rifle with heavy recoil but I think that you may find it can substantially interfere with where a double rifle puts the bullets.

The accepted way to test the regulation of a double is to shoot it standing in the shoulder with the back of your front wrist rested on a tall soft rest. One barrel and then the other without delay.

That duplicates as well as possible how it will shoot when you use it in the field whilst adding an element of steadiness.

Parabola