Cronograph inaccuracies

[RDavies]

I,m starting to see whay some people are able to get some realy high velocities from their cartridges, compared to what other people are getting. (not counting the keyboard marksmen, whos groups shrink and velocities grow at the computer).
At Brisbane I bought a new crony to try to sort out a new cartridge I was trying. I found I was getting 130 fps lower than expected, so I kept bumping up powder loads till cases got sticky. Then I checked some existing barrels and decided the crony was wrong. I took it back and exchanged it for a new one and this one gave the sort of velocities I was expecting. (130 fps higher than the other one).
This morning I went out with some mates to cronograph their 308 hunting rifles and found they were getting very high velocities. We ended up running 2 of my cronies and one fo theirs inline and found my 2 (one 20 years old, one 2 month old) were giving reliable readings, while the other new one (a freinds) was giving 140 fps higher readings on an over cast morning.
If I diodnt take the dodgy one back, there would have been 270fps difference just in cronies.
I wonder how many people get caught out with this, trying to bump up loads to get what everyone else is getting?

[flatlina]

I've had that with the Chrony brand but never with the CED Millenium, especially if using the infra red screens.

Regards
john

[TOM]

With equipment like this you really do get what you pay for. Much like scopes, powder scales etc, your cheap and nasty will give you a false sense of security and no doubt cause you more problems than spending the extra money would have. At the end of all the time and frustration fooling with cheap "shit" you can really appreciate the good stuff when you bite the bullet and go and buy it, or at least get to use it.

The difference you saw today Rod can be the difference between a super and sub sonic bullet at 1000 yards. That adds a bit of perspective to the importance of accurate and reliable equipment needed for long range target shooting dont it.

[RDavies]

I agree most times that you get what you pay for, but the 2 cronies I have which are working well, are cheapies, one of them 20 years old, with no more sun screens. The most expensive crony I have bought is gathering cob webs as is very inconsistant in reading shots. I would like to spend some bigger bucks and get an Oehler sometime.

When I have my 2 cheapies (different brands, 20 years age difference) inline together, the biggest difference in 40 shots was 4 fps.

[AlanF]

AFAIK there's no readily available and accurate way of testing chronographs for absolute velocity. Even if you get an Oehler, who's to say its velocity values are truer than an F1 Chrony? I have an F1 Chrony and a CED M2 (now with IR screens). Before the IR screens, I used the F1 and M2 in series, and in a session they usually maintained a constant difference. However the difference one day might be say 10 fps, then another day it could be 60 fps. Since I got the IR screens for the M2, I use it without any corroboration from the F1, because I've come to trust its shot to shot accuracy. However it could be measuring all shots 50 fps high or 50 fps low - there's no way of telling . When I quote velocities on these forums, its the M2 IR velocities, and they're probably conservative, because the IR screens give lower velocities than the sky screens on the M2, which in turn are lower than from the F1. Hmmm - maybe I AM actually getting 2950fps before the primer pockets stretch. !

Alan

[Fireman_DJ]

I think it the point should be made, why try to duplicate the velocity that someone else's gun likes?

I have a chrony, but as long as it's pretty consistent shot to shot and within 200fps of the actual, then it's accurate enough. It'll tell me if my shots are at a consistent velocity and be close enough to calculate ballistic charts.

[johnk]

Won't there be a correlation between measured velocity, field ballistics & ballistic (prediction) tables?

[Barry Davies]

Fireman,
How do you know the "actual " speed

An error of 200 fps can mean the difference between getting to 1000 or even 900 yards accurately or not.

Barry

[Fireman_DJ]

Assuming a 300yrd zero for a 1000yrd target I got the following numbers out of PointBlank.

2600fps = -511.5 inch drop
2650fps = -491.7 inch drop
2700fps = -472.6 inch drop
2750fps = -454.6 inch drop
2800fps = -437.1 inch drop

That's a max error of 74 inch at 1000yrd. On a 94 inch target. With someone at the target guiding you in, they could tell you if that's too high or low and the second shot you'd be on paper.

If you want your ballistics program to put the bullet inside a 5 cent coin at 1000, then most ballistic programs aren't that accurate enough even before you add your data.

In my opinion, that's close enough for me to document my rifles performance and build up a range chart for my specific rifle.

I should also note that the round I used for the above numbers was a 180gn hunting round. The target ammo we use has a better BC, but those figures are on my laptop.

[AlanF]

DJ,

Our targets are 6 feet (or 72 inches) high. If the correct velocity reading gets you into the centre, then a vertical error of just over 36 inches will get you a miss (either above or below). On your drop figures, 36 inches equates to about 100fps. You also have other inaccuracies such as head/tail winds, atmospheric conditions, elevation accuracy of the rifle/ammo/shooter etc. which further compound the problem. So I'd suggest you'd need to know your velocity to within 50 fps to be reasonably sure of hit at 1000, and that would be after your cold barrel/fouling shot(s), which usually go low.

Alan

[Fireman_DJ]

Alan, my point is more so not to use the chrony to get pin point accurate. There's too many other variables that will affect it. Hence it doesn't need to be perfect, as long as it's consistent.

Ie, finding out what velocity is stable and your barrel likes and getting close to the target from where you can document the actual data for your rifle.

[Norm]

I use my chronographed velocity as a guide only. It sets a velocity benchmark and helps provide an indication of ES on my loads. However I do not rely on chronographed numbers for ES.
Even though my chronograph is reasonably accurate. I find that it does give variable readings with the same load under different light and temperature conditions.
Chronographs come with a manual that states the error factor in +or- numbers. These can be overstated.
For ES of my load I shoot them out to near transonic and check for vertical spread.
For B.C. data I tend to use a base line bullet of known B.C. that I am familliar with. Litz test data from his edition II book is also useful as a guide.
With the test bullet I shoot it over the chronograph with the base line bullet to match velocity by changing the powder charge. Rifle zero offest is also checked at the same time.
Then both bullets are test fired against each other out on my private 1000 yard range.
This method has produced very reliable results for me.

[RDavies]

Where I think people can get caught out is when they try to get to known nodes. ie, try to get up to a known higher node such as 2950 fps in a 284 or 6BR for example. (or even a standard node if the crony is reading low enough).
Some use the crony for initial load development at shorter range until they get good groups, so they wont know the speed is way down by checking comeups. This is what happened to me. I knew I wanted 2820 fps, so I kept upping the powder load to try to get to it. When cases got real tight and primers cratered, I knew something was wrong, so I rechecked the crony with another known gun.

[AlanF]

Where I think people can get caught out is when they try to get to known nodes. ie, try to get up to a known higher node such as 2950 fps in a 284 or 6BR for example. (or even a standard node if the crony is reading low enough).
Some use the crony for initial load development at shorter range until they get good groups, so they wont know the speed is way down by checking comeups. This is what happened to me. I knew I wanted 2820 fps, so I kept upping the powder load to try to get to it. When cases got real tight and primers cratered, I knew something was wrong, so I rechecked the crony with another known gun.

Rod,

I think people have to find their own nodes and have a reliable way to measure the velocities with their own chrono. Your theory about 2820/2950 fps nodes is credible, but its never going to be easy to get those velocities precisely using a chrono. What I do is rely on the M2 with IR screens under a roof with no direct or reflected sunlight, and that is my "standard environment". So if I discover a good load at the range, I then measure its velocity at home in the standard environment. Then if I change any of the components and want to get back to the same velocity, I work up the load at home, again measuring the velocity in the standard environment. The only time I use the chrono at the range is for ladder testing. I seem to remember you haven't been happy with the M2 and IR screens. Have you tried it under a roof? (I use a small poly tarp).

Alan

[RDavies]

Alan, I havent used it under a roof, but have tried it in the dark and dusk. Same thing, not reading very often.