If the scope is shifting in its mount ever so slightly, it tends to cause vertical stringing in the POI. Since you said the bullet was jumping all over the place, and you mentioned that mount and rings were torqued to spec, we can probably rule those out.
Assuming this is not your first rodeo, let's throw out all the obviously "noob" mistakes.
That leaves 2 likely causes:
1) Shooter needs to manage 300WM recoil better
or
2) Scope is screwed and reticle is jumping all over the place
I agree with posts above. Throw out the lead sled. I have a 9.5lb unbraked 300WM. The first time I shot it, my shots were all over the place, till I learned proper fundamentals and manage recoil better and more consistently. Get a good front and rear shooting bag, get into a good shooting position, manage your recoil consistently and see if it gets better.