Mobuck has it right. When I use mine,,, I do not add too much weight. Yet,,, when I have a guy who wants a 375, or an bigger caliber set up for hunting, I use the sled. I usually have to either mount a scope or re-mount it correctly. Then I have to zero it & shoot it in. I also get down & behind my sled, sorta "into it". I don't try & fix it rigidly to where it doesn't move at all. I let it rest against my shoulder, and let it rise up on the forend. By using it more like a normal rest, yet, allowing the weight to help reduce the felt recoil to my shoulder,,, I can shoot a lot more w/o the abuse to my shoulder.
I've never had a stock break or anything.
I also make the owner shoot his gun, on a normal rest, to final zero it.
It's a tool, and when used properly,,, works quite well.