chronograph accuracy

[flintlock.50]

I've had a Pro Chrono Digital by Competition Electronics for a couple of years. I get good repeatability on my loads, but they I can't seem to match what the books say or what other folks post for their loads. For example:

.30-06
Rem 700 (22 inch barrel)
165 gr Hornady SSTs
58.0 gr of IMR 4350
Nosler cases
CCI large rifle primers
average MV 2771 fps (10 ft from chrono)

Nosler Accubonds in the same load and cases average 2749 fps

I got 2863 fps in another batch of Nosler cases but they were 20 grains heavier than my current batch so their case capacity was considerably smaller. They never showed signs of over pressure, but I only got 3-4 shots from them before they started showing signs of incipient case head separation.

So here's my dilemma. I have a Leupold VX III scope with the CDS dials. I'm about to order a dial for my 165 grain load. However, if my MV is off the dial won't be correct. What is typical for 58 grains of IMR4350 behind a 165 grain bullet? Are my numbers credible?

[PawPaw]

I'd say your numbers are fairly credible. Factory ballistics charts (I just looked at Federal and Remington) show 2800 fps with 165 grain bullets in .30-06, but you must remember that they're using test barrels in rigidly controlled conditions.

Your rifle is different. My rifle is different. They're all different and will turn in different velocity numbers with the same ammunition.

Trust your testing with your rifle and ammo, with your conditions.

[wogpotter]

Lets make a couple of assumptions, just for the sake of argument.

Your chrono start screen is not AT the muzzle but is about 10~15 feet away.
That gives you "velocity instrumental at (x) feet" not muzzle velocity. That costs you a tad in measured speed. The factory does calculations allowing for this & giving them "corrected muzzle velocity".

Your barrel is not the exact same length as the one used in the test.
Check this out I went nuts once trying to figure out a published velocity till I read the barrel length, 15" this was for a revolver

Altitude is probably not the same, Humidity & air pressure is probably a little different. Powder, primer & brass/bullet lot #s are definitely not the same.

Add a few things like this together and the 10FPS here & the 25 FPS there add up to maybe a hundred FPS or so. Basically if your data is repeatable & consistent. No unexplained 500FPS "flyers" once in a while & so on your data is probably more accurate for your gun & load than the factory data from their gun applied to your equipment.

[Boberama]

You can correct for muzzle velocity by using an online calculator and the ballistic coefficient of your bullet.

So if the chronograph is 10-15 ft away, you can make up the small difference.

Try JBM's calculator, it has an input for chronograph distance.

[flintlock.50]

I use JBM's ballistic calculator lots. It's pretty good.