Earlier today I ran across a thread here on LRH about Chronographs and I'm hoping someone could help me sort out a problem I'm having with my Beta Master Shooting Chrony, a rifle and load combo. My question is how to prove the calibration of the chronograph? I recently did shoot across 2 other chronographs on 2 different occasions. First time, all were within a few FPS of each other. The second time, a few weeks later, there was a very wide difference – off by hundreds of FPS. My Chronograph was the slowest. A Tech at Shooting Chrony suggested that I could get some factory ammo with a "known" velocity, shoot it and see the results. Seemed reasonable, so I did just that with the rifle in question (a 7 mag) and several others in different calibers. While each was somewhat consistent (factory ammo), they were not what was advertised and off by more than I would have expected. Some were faster while others were slower. My conclusion here is nothing proven or dis-proven with this test. Could anyone help?