What if I measure the depth of the chamber with a caliper?
For it to take a 70mm cartirdge do I need greater length in the chamber?
the chamber is made for the shot when fired, the 65 becomes roughly 70mm that is why the 70 fits
shooting 70mm in a 65mm gun can cause higher gas pressures and whatnot, the most likely thing that will happen is that yo won't get the spread/coverage you wold excpect, but I can't recommend it. the least that will happen is that you will shot your gun loose de to the higher gas pressure
it will be hard to measure because the chamber gradually cones out into the barrel.