My Velocities are Varying by 600 fps!

[Jerry D]

I'm loading 52.0 grains of IMR 4350 in my .30-06 shooting 168's and my velocities are varying by 600 fps. All my rounds are loaded to the same overall length + or - a few thousands and my powder chargers are accurate to within 0.1 grain.

I've chronied many rounds... about 15 or so. Most came up at just over 2500 but a few actually more than a few, like 5 of them came up at 1900 fps. There were none between the 1900 and 2500, it was either 1900 or 2500.

Anyone have ideas? Is my chrony bugged? I never did have the sunshades on and it was sunny... could that be the cause?

 

[ranger335v]

"I never did have the sunshades on and it was sunny... could that be the cause? "

Yep.

 

[helg]

I would add that this could be muzzle flash.

 

[Kernel]

Fresh battery for the chrony?

 

[Runningman]

Also just to add to the replies a I've got some strange readings if the chronograph was two close to the muzzle blast. Seems like they work best if the chronograph was 12 feet from the muzzle. A new battery might be a good idea.

 

[jpwilly]

Chronographs work best at 15ft+ and you should uses sunshades on sunny days.

 

[USSR]

Direct sunlight on the sensors cause erratic and impossibly high readings. The two diffusers that come with the chrony are not enough. Tape some translucent paper or plastic between the two diffusers to keep all direct sunlight out.

Don

 

[MCMXI]

When I first started using a friend's chronograph (subsequently purchased from him), I enjoyed months of trouble-free use. However, as the winter turned into spring, I began to get numerous Err1 and Err2 messages and erratic data. I always used the plastic sunshades (diffusers) but realized that they simply weren't large enough. I ended up making a pair of wider aluminum sunshades and used threaded inserts so that I could bolt them to the chronograph's steel housing rather than attach them to the sensors. I also added a piece of foam board with a slot just slightly wider than the sensor "eyes". At 100 yards I can see all of my 5' x 1.5' target frame so I know that the bullets are passing directly over the sensors. With the slot in place, it's virtually impossible to shoot any part of the chronograph .... if I can't see the target I don't pull the trigger. Since making the "upgrades" shown below, I've had 100% reliable data in very bright (sunny) conditions. I also added a bubble level which helps a lot when setting up at the range.

 

[Walkalong]

Excellent job 1858!

 

[MCMXI]

Walkalong, thanks ... it took a bit of work but it was definitely worth it. I haven't put so much as a nick in the foam board yet so the "window" works well too.

 

[MovedWest]

1858 - I must say I have Chrony envy! Sweet set up!

-MW

 

[USSR]

Hell of a good job, 1858!

Don

 

[MCMXI]

Thanks guys .... I appreciate your kind words. Here's a shot of the bubble level and a couple of the threaded inserts in the bottom of the sheet metal housing. I used two inserts per sunshade, and they're offset (front to back) to distribute the load. It only takes a few seconds to install/remove the sunshades.