Chronograph accuracy

[Cheyenne 1]

How accurate are chronographs? For some reason whatever I load never seems to be as fast as everyone elses. The worst ones for me seem to be the ones that drive them faster eg. 7mm stw, 257 stw AI loads. It seems that after 3500 fps they get like they max out. The odd time it will be what I think it should be then after that it won't. Does light/sunlight/shadow make that much of a difference?
thanks

 

[rscott5028]

You can check the accuracy specs that came in the box with yours.

Chronographs are generally pretty accurate ...that is, when they get a good reading.

There are a number of variables such as those you listed that will distort your readings.

One that you didn't list is muzzle blast which is possibly your issue if its occuring at the higher velocities since the muzzle blast is likely to be increasing as well. You can try moving the chrony farther from the muzzle to offset it.

I see a lot of load data, stated velocities, and accuracy statements posted on the Internet that don't quite add up. But, the only thing that counts is what your gun is doing. So, work up a safe load and use the chrony as a relative measure for what's going on. Then, shoot your rifle out to longer distances to confirm your drops.

I'm more than happy to give up a few fps for better accuracy and barrel life.

-- richard

 

[Bart B]

Chronographs are very accurate if properly set up and screens spaced correctly.

There's three reasons why one person's velocity for a given load will be different that everyone else.

1. The powder and primer lots are different. Each one performs a tiny bit diferent than others. You may find that going from powder lot A to lot B requires half a grain or more of powder to get the same velocity. A 50 fps difference between lots A and B with the same charge weight is normal. Powder making is not an exact science. Primers have the same differences but they're just smaller.

2. The barrel you use may well have different bore and groove diameters as well as chamber throat shape. That's enough to easily have a 150 fps difference than someone elses barrel. Expensive pressure and velocity test barrels are made to SAAMI chamber, bore and groove dimensions that are very exact. Factory and aftermarket barrels' bore/groove diameters have quite a spread; 30 caliber ones tyically range from .2970/.3065 up to .3010/.3090 which is different than the .3000/.3080 SAAMI spec for most 30 caliber cartridges. Couple this with 30 caliber bullet diameters ranging from .3065" to .3092" and it's easy to understand why velocity comparison/matching is not all that easy.

3. Us humans don't hold shoulder-fired rifles with the same tension against our bodies. There can easily be 100 fps difference between what two folks get with the same rifle/ammo when fired from a bench with the rifle resting on something and held to their shoulders. Standard SAAMI test barrels are held in a heavy machine that doesn't move in recoil; they always shoot bullets faster than us humans would from shoulder fired rifles.

This all adds up to

 

[rscott5028]

Chronographs are very accurate if properly set up and screens spaced correctly.

There's three reasons why one person's velocity for a given load will be different that everyone else.

1. The powder and primer lots are different. Each one performs a tiny bit diferent than others. You may find that going from powder lot A to lot B requires half a grain or more of powder to get the same velocity. A 50 fps difference between lots A and B with the same charge weight is normal. Powder making is not an exact science. Primers have the same differences but they're just smaller.

2. The barrel you use may well have different bore and groove diameters as well as chamber throat shape. That's enough to easily have a 150 fps difference than someone elses barrel. Expensive pressure and velocity test barrels are made to SAAMI chamber, bore and groove dimensions that are very exact. Factory and aftermarket barrels' bore/groove diameters have quite a spread; 30 caliber ones tyically range from .2970/.3065 up to .3010/.3090 which is different than the .3000/.3080 SAAMI spec for most 30 caliber cartridges. Couple this with 30 caliber bullet diameters ranging from .3065" to .3092" and it's easy to understand why velocity comparison/matching is not all that easy.

3. Us humans don't hold shoulder-fired rifles with the same tension against our bodies. There can easily be 100 fps difference between what two folks get with the same rifle/ammo when fired from a bench with the rifle resting on something and held to their shoulders. Standard SAAMI test barrels are held in a heavy machine that doesn't move in recoil; they always shoot bullets faster than us humans would from shoulder fired rifles.

This all adds up to

Bart --

You left out reason number 4...
Many of us tend to stretch the truth or selectively report the facts.

-- richard

 

[Scot E]

Bart --

You left out reason number 4...
Many of us tend to stretch the truth or selectively report the facts.

-- richard

LOL! That one should probably be moved to the #1 spot!

 

[Bart B]

Bart -- You left out reason number 4...
Many of us tend to stretch the truth or selectively report the facts.

Selective fact reporting is only limeted to accuracy claims; never ever mention groups larger than the smallest one ever shot.

Surely nobody ever claims higher bullet speeds than they really travel at, do they?

 

[Cheyenne 1]

Thanks for the help.
The muzzle blast was the one that I was thinking about. The chrono rocks quite a bit when I shoot.
To make things as equal as possible I have been using a lead sled with 50 pounds of weight.
The other problem I have is the last rifle I was loading for I can't find any load data. It is a 700 remington chambered in a 257 STW ackley improved. I was just using the regular 25stw loads and seeing what they would do and kind of bumped it up a couple of grains.
After a point it seemed stay the same speed and then all of a sudden it was faster. That is why I was questioning the chrono.
Thanks again

 

[HARPERC]

It's been awhile, and I don't remember which manufacturer it was now, but the quote was "we've had our chronographs returned for inspection/repair, always for too slow, no ones ever complained about having one that was reading too fast"......

 

[rscott5028]

Selective fact reporting is only limeted to accuracy claims; never ever mention groups larger than the smallest one ever shot.

Surely nobody ever claims higher bullet speeds than they really travel at, do they?

That's why I only shoot one shot groups. ...never larger than caliber size.

 

[Bart B]

That's why I only shoot one shot groups. ...never larger than caliber size.

They'll be as small as possible if you use barrels whose groove diameter's 2 thousandths smaller than bullet diameter. Shaving a couple thousandths off any group is an improvement!