I have wondered how well these things worked for some time and finally got one. Wish that I had not waited so long - the little gadget works slick and beats hell out of the guess work that I used to use to determine max O.L. You just screw a dummy cartridge case onto a threaded hollow stem, drop a bullet into the case mouth, insert the case into the chamber and push the bullet into contact with the lands by sliding a plastic rod up through the stem and cartridge case. Way easier to do than to explain. When the bullet makes contact you lock the push rod with a setscrew and withdraw the unit from the action. The bullet might stick in the lands, no deal as you just pop it out with a cleaning rod dropped down from the muzzle. Then put the bullet back into the case mouth, it drops down onto the locked push rod and you have your exact O.L. determined at the OGIVE, not the bullet tip. You measure the base to tip length or actual overall length with a caliper, the stem is cut-away to allow you to center your caliper so this is easy and accurate. Sure is a neat tool, not expensive and has to work. I believe Sinclair sells one also.