Higher than normal?

[johnnyk]

Set my chronograph up Wednesday to check a new load in my 7Mag. I decided to check the zero on my .243 while I was at it. The load for the .243 was the 95gn Ballistic Tip ahead of 44.0gn of VihtV N560/BR2. I checked it in Nov09 and it averaged 3088fps, so I was suprised to see it shooting above 3200fps. The first three shots were 3271/3278/3247 (3265fps). The temperature was 51deg. I thought, Hum?
I went to the 7mag next. The previous load had been with a 150gn Ballistic Tip with 63.5gn of IMR4350/9.5M and I was getting 3234fps in Nov09. I worked the load last week up to 64.0gn of IMR4831/9.5M and accuracy was great. The five shot string was 3265/3301/3281/3336/3321 (3300fps). The first blame was put on the chrono's battery.
I went back to the .243 and shot a 6 shot string. They were 3191/3206/3217/3184/3230/3183 (3201fps). I thought WOW, wouldn't that be great. I know something is not exactly right but haven't figured out what that might be. The distance from the muzzle to the chrono was 10-12ft as normal. I don't think it was the temp as that would cause lower velocities. The only thing I can attribute these higher velocities is the angle of the sun. It was approx. 5:00-5:45PM, which at that angle and time it's almost down behind the mountain here. I check the batteries voltage and it was 9.7volts, more than enough. In the past a low voltage has resulted in very low voltages during range sessions. What do you guys think? JohnnyK.

 

[Moman]

Johnny, I have had similiar results like this that were due to lighting differences. The worst for me have been the days that are partly cloudy. I guess it's just brighter on those days and the amount of light changes. My best, or what I assume to be most accurate results have been on overcast days when the amount of light stays constant.

 

[outofayr]

I've also seen this once a couple years ago with my Shooting Chrony - you didn't mention what brand yours was? It was on the order of 200 fps high. Next time out, it was back to normal.

May have been the light, but it may also have been the chrony maybe wasn't opened up all the way so that it was flat. This would shorten the distance between the two "windows", thus causing a shorter time of flight between the start and stop windows. The chrony computes fps by using a fixed distance, so a shorter distance = higher velocity.

Since that time, I'm anal about making certain the chrony is FULLY opened and flat, and haven't had that problem again...

Hope this helps,
Brian

 

[AtownBcat]

I 2nd what outofar says. I also have a shooting crony, and the piece that connects the crony to the tri-pod had kept the thing from opening all the way. I had higher than norman readings.

 

[Moman]

You guys are probably on to something with the Shooting Chrony. Another thing I've really been focusing on is taking the time to make sure the height is adjusted right. I've heard that you can get some variation if you are shooting over it at different heights. Maybe not as critical, but trying to keep everything consistent.

 

[johnnyk]

A-hah! That was it. It is in fact a Shooting Chrony and I noticied that it wasn't open all the way and flat like it usually was. It wasn't much but enough that I also thought the skyscreens looked a little closer at the top. I tried to flatten it out but it didn't seemed to move, not sure what was going on there. What a brain fart! Thanks guys.
Darn, now I'll have to shoot my rifles again! JohnnyK.

 

[AtownBcat]

you have no idea how hard it is to be this smart.....

 

[johnnyk]

wrong....