Chronograph question

[Newsportshooter]

Hi Guys
So my question is this, i have a caldwell balistic chrono. Not the most expensive piece of kit on the market. But, i now have it set up so that it always gives a velocity figure.

So heres the question, does the fact that it is giving a velocity figure for each shot I fire over it mean that the figures are accurate?
Tbh i was just happy to get it reading every shot, now Im wondering how accurate it is.
Any way to have them tested/calibrated??

I suppose i could set it up with s labradar to the side and compare the figures, then theres mv truing against predicted drop from an app.

Any thoughts chaps?

[Daryll]

So if you use a Labradar to compare, how do you know the Labradar is accurate..?

Everything has a variation... your chrono may be accurate within 1%, (Caldwell actually claim 0.25% tolerance...!!), but at 3000fps, thats a 30 fps variation... and if you compared it with another chrono, you could end up with 60fps difference, but they're both within their manufactured tolerance..

Unless theres any physical damage to the chrono, assume its accurate.

[WelshShooter]

In all honesty, how precise do you need to know the velocity? From a simplistic point of, it is almost impossible to achieve exactly the same velocity for every shot, there will always be some level of spread even if low, single figures. Then you have a small amount of error in the measurement system.

Measuring velocity during load development (eg Satterlee method) can help pinpoint nodes in your velocity strings and you settle on a "flat point" to get less charge in velocity if your charge weight varies slightly. Once you've settled on your load shoot and measure around 30 shots. Take the mean and standard deviation of your velocity.

Calculate the mean plus 3 times your SD, and the mean minus 3 times your SD. Assuming your data follow a normal distribution (ie a bell shaped curve) then the velocity for 99.7% of all your future shots will fall in this spectrum (assuming shooting conditions remain the same, of course). I'd then figure out whether this velocity range will really affect me or not.

I use a Magnetospeed sporter and when developing a load for my 6.5x47 Lapua I achieved an optimal load with an average velocity of 2,929fps and an SD of 6fps. My plus/minus three SD range is then 2,911fps to 2,947fps. I mostly shoot 100m now so that spread is more than acceptable. I used shoot at informal longer range targets (Sennybridge F range) out to 1,300 yards with this rifle and load and in all honesty, there was much more horizontal spread because of windage than vertical spread.

[Newsportshooter]

Thanks chaps, just wondered if the equipment could fall outside of tolerance and if there was a way to calibrate. But these answers have given me more to think about.

Cheers

[Pete]

How accurate do you need it to be? I wouldn't beat myself up looking for sub 10fps shot to shot deviation......that way madness lies.
I'm more concerned with group size, and paradoxically, the two rarely go together.

Pete

[ukrifleman]

I compared some 7.5x53.5 cast loads for my mod 1889 Schmidt Rubin, using my Chroney F1 and my clubs lab radar and got
an average m/v of 1862fps/1867fps respectively.

Pretty happy with that.

ukrifleman