I have tried similar with a M12 bolt and stack of washers, however I suspect there wasn't enough stretch in the bolt/washers to make it accurate enough, as readings seemed to vary quite widely.

My thought this morning was to get a bit bar, drill a row of holes to accept a row of nuts/bolts, torque them down at increasing torques, then see what the gun can slacken. What I really need to know is how tight a bolt the gun can slacken, as I'm wanting to test fi I can replace it with some form of high torque motor/gearbox.