So I have an F1 Chrony and many times it will give an error. When I first got it I had it set too close to the muzzle on a tripod. I shot my 30-06 through the window and it rocked the thing back and almost fell over. OK move it back. I now have it usually at 20 feet but have set it at 100 yards. Too close and I have got odd readings like 3500fps from 168 gr .308.. yea that can't be correct. Less errors farther out. Other times it was partly cloudy or the sun was at the wrong angle. I would get ERROR on the screen on a few shots I removed the sun shades from above the chrony. And just shoot between the sticks. No more errors. There was one guy that took a piece of plywood about 2 foot by 2 foot and painted it white. Mounted the chrony upside down above the white plywood placing the plywood on the ground. No more errors. If your F1 is finicky or other chrony you may want to try some of the above.