Chrony question

[Ray P]

October third, I fired half a box each of Armscor and Blazer 45ACP (230g FMJ) through my new chrony. Data: (in feet per second)

Armscor: hi: 1149 lo: 837 Ave: 981 SD: 83.4
Blazer (old): hi: 1011 lo: 968 Ave: 990 SD: 12.5

Then I did a few more boxes of ammo today (S&B, Wolf, Fiocchi, etc; all 230g FMJ 45 ACP). I noticed substantially smaller velocities, and significantly less data point spread. So I tried the remainder of the Blazer, and got completely different results from last week.

Aguila (typical): hi: 884 lo: 835 Ave: 860 SD: 12.0
Am Eagle (typ): hi: 883 lo: 846 Ave: 861 SD: 8.7
Blazer (new): hi: 862 lo: 823 Ave: 833 SD: 11.0

The days were very similar in weather; sunny, little to no wind, temps in the mid 50's. My chrony was set up the same; 3 ft in front of my shooting position, with both screens down for direct sunlight.

Any thoughts?

[Ray P]

Please excuse me if this is off-topic, but reloaders are the people I believe would be most familiar with problems in the use of the chronograph. Fyi, mine is a Chrono-Tech.

[mbott]

The days were very similar in weather; sunny, little to no wind, temps in the mid 50's. My chrony was set up the same; 3 ft in front of my shooting position, with both screens down for direct sunlight.

Ray, I've always felt that 3 ft. is just a tad close for consistent results. My preference is 10 feet for both rifles and pistols. I also believe that you want to use the screens with strong, direct sunlight. That gives the sensors a consistent background in which to operate.

--
Mike

[Ray P]

Thanks, Mike. I will move it back in future use. I did run some more rounds for info, and I think I found the cause of my high velocities: Direct sunlight on the ammo in loaded magazines waiting to be shot. This is supported by two things:

1. Ammo velocities started out normal (approx 850 fps), and steadily climbed (to over +P levels at 1100 fps) through the 25-round course of fire, the amount I had pre-loaded before I started shooting each string.

2. Strings (of 25 rounds) shot later in the day, when the shooting bench was in the shade, had much more reasonable results. On subsequent days velocities stayed in the expected range when I kept the ammo out of direct sunlight.

Using the data from the later courses of fire, pretty much all the ammo I tried, US and foreign, had standard deviations in the high single digits to mid-teens.