One afteroon as the load testing concluded, I decided to run a few of my hot .357s over the Chrono again. What had been a nice, stout 1375 fps load using a Speer 146-gr jswchp, was now doing 1800+FPS!!!
Sure, it was hot that day, but recoil, blast and muzzle rise still appeared as before.
On shot #4, it went up to more than 2,000 fps, and I could see that the 50-yard point of impact was still unchanged. What's up???
Then I noticed that my back was to the sun and the shadow of my revolver, at 6 feet back from the first screen, was passing only about a foot below the screen. I aimed lower and closer to the screens, and got a 1500 fps reading.
Problem??? The gas cloud from firing the shot was casting a shadow over the first and second screens in a VERY short time, far shorter than the bullet would. Eliminated muzzle blast bouncing the diffusers as a potential cause by turning the direction of fire 90 degrees--all other factors being the same, I got nothing but high 1300s for the remaining readings.
I vote for Chrono error, even though lead bullets usually shoot faster than jacketed, with equal powder charges.