Chronograph Question

[joeb33050]

I get "funny" results when chronographing, maybe suspicious. I get velocities of 253.7 fps and 4512 fps in the middle of 1500 fps strings.
I get a lot of first-shot-high-velocity readings.
Wednesday I got this:
1460
1471
1462
1466
1455
1465
1546
1564
1556
1456
1463
1530
1592
1468
1455
1473
1467
Pact chronograph, light diffuser items installed, it was not the ammunition.
Any idea what's going on?
I chronographed 8 loads on wednesday, all the rest looked ?normal?
joe b.

[JSnover]

Make sure the screens aren't too close to the muzzle. Your machine might be clocking clumps of lube from the boolit.

[redbear705]

I was chrono'ing loads a week ago and was getting wild deviations so I moved the chrono out two more yards (6yd total) and all is well.

I happened to notice that when the wind was blowing I could get normal results and when it slowed down the results were off.

...so......I believe the smoke or Lube ( as suggested before) was getting there first causing variations.....heck who knows maybe it was unburnt powder? :)

JR

[hyoder]

Shadows moving across the screens seem to confuse things also. If you set up under trees try to get full shade or full sun.

[Pepe Ray]

You claim "it was not the ammo".
You say this because? You weighed each charge?
FFT,
Pepe Ray

[454PB]

I agree, keep the chronograph as far as possible from the muzzle.

[tall grass]

I've had trouble when the sun got low, clothspined a shirt on the sun side of the defuser rods and then it worked.

Jim

[joeb33050]

You claim "it was not the ammo".
You say this because? You weighed each charge?
FFT,
Pepe Ray

Dribbled/weighed
joe b.

[BeeMan]

If you are dealing with boolits, try coloring the exposed portion of the projectile with a black marks-a-lot or sharpie. I don't recall where I learned this but my Oehler model 33 gave frequent bogus readings with cast projectiles. A black sharpie did the trick. I don't remember having issues with jacketed, so it may be the color difference between jacketed and cast is significant with some sky screens and light conditions.

BeeMan

[Jon K]

joe33050,

It's batteries, when my PACT act up like that, fresh batteries- problem solved.

Jon

[AZ-Stew]

Dr. Ken Oehler had a column in Shooting Times for about a year. One of the articles he wrote had to do with your problem. There are a number of things that affect chronograph accuracy.

Try to do your shooting when the sun is overhead, rather than low in the sky. Light reflecting from the bullet surface can cause false readings.

Use the diffusers over the screens. Apparently, the photosensors have a hard time seeing the bullet pass against the blue of the sky. The diffusers give a white background for the sensors, which makes the bullet a higher contrast object against the background, in turn making it easier to detect.

Shoot as close to the top of the screens as possible without shooting them. An inch or two over the sensor will give more accurate readings than shooting 4-6 inches above them.

There were others, but those techniques should eliminate 90 percent of your problems.

Regards,

Stew

[standles]

Upgrade to IR screens and not worry about cloouds, passing shadows, little gremlins etc. etc.


Steven

[Alchemist]

BeeMan,

The sharpie solution is in the Oehler 35P owner's manual...the 33 is essentialy the same chronogragh...without the printer or second (proof) channel.

There is also a section on "glint"...the clear sky false trigger phenomemon in the 35P manual.

[joeb33050]

I have the screens at the ?correct distance below the bullet. Both lower and higher on the screens makes more funny readings.
I try a new battery when things get funny, keep new battery in the kit.
I'm interested in anyone else's experience with funny chronograph readings.
Wed, 4/16/08, 45/70, 22/SR4759 dribbled to weight-velocities SORTED
1144
1157
1166
1182
1183
1184
1184
1185
1186
1196
1197
1200
1206
1209
1218
1236
1282
The 1282 looks like a stranger to me.
I try to chronograph a minimum of 15 shots, I know that's not enough, but cast bullet lot size limits me.
I don't edit out or trim any readings.
I can't decide if these strangers are just part of the distribution, or are chronograph errors.
I get the odd "372.9" reading, do throw that out.
The standard deviations in powder position sensitivity testing, 37 tests, average SD = 23.6 fps
smallest SD 9.8 fps, largest SD 46.0.
Anyone else done any lengthy chronographing testing?
joe b.

[Pepe Ray]

Joe;
Are these last #'s arranged by you or "as fired"?
Pepe Ray

[runfiverun]

how do these look on the target?
are you getting any vertical stringing, this would bear out strange velocities.
it could be a powder position thing, or your chrono is having a wiring problem.

[joeb33050]

Joe;
Are these last #'s arranged by you or "as fired"?
Pepe Ray

As the post clearly says, velocities have been SORTED.
joe b.

[joeb33050]

how do these look on the target?
are you getting any vertical stringing, this would bear out strange velocities.
it could be a powder position thing, or your chrono is having a wiring problem.

I started this experiment chronographing shots with powders in various positions, and shooting 5 shot groups. I shortly gave up shooting groups, there was just too much going on-writing down vel., moving the rifle, the constant erratic wind, etc.
There is no connection between velocity and bullet hole elevation. These shot the middle out of the target.
The whole experiment is about powder position sensitivity, see below under CAST BOOLITS place.
I can't tell if it's the chronograph or what I should expect by way of variation.
There must be others here who have chronographed long strings >12? of shots.
Others write of SDs of 10 or less, mine are much larger.
joe b.

[oso]

With my Chrony I use cardboard shades to keep direct sun off the eyes without blocking the vertical view.
I also try to keep ammo temp the same, not letting it get hot in the sun or interrupting cadence so a round doesn't sits longer than others in a warm chamber.
Don't forget to consider insects flying and crawling around at inopportune moments.

[runfiverun]

joe
i didn't realize this was part of your test, i have been following it
i just have had a variation in vel and it showed up as stringing on the target, and the chrono
confirmed it for me with velocity variations.
i try to run strings of about 20 at a time mine keeps track of them for me so i can go back and
follow the string after the fact.

[joeb33050]

joe
i didn't realize this was part of your test, i have been following it
i just have had a variation in vel and it showed up as stringing on the target, and the chrono
confirmed it for me with velocity variations.
i try to run strings of about 20 at a time mine keeps track of them for me so i can go back and
follow the string after the fact.

Do you get these kinds of strange velocity variations from time to time? What kind of SDs do you get with long strings of shots?
Thanks;
joe b.

[joeb33050]

With my Chrony I use cardboard shades to keep direct sun off the eyes without blocking the vertical view.
I also try to keep ammo temp the same, not letting it get hot in the sun or interrupting cadence so a round doesn't sits longer than others in a warm chamber.
Don't forget to consider insects flying and crawling around at inopportune moments.

I shoot between ~9:30 AM and ~2 PM. Try to keep ctg from heating up in the chamber. No sun on ammo, in the shade. Maybe it's bugs.
joe b.

[runfiverun]

every once in a while i'll get a freaky number, it seem to come up when my
memory is just about full.
and it is usually the ffirst shot that is goofy like the thing is dozing and i scare it.
if it is only one or two per 20 shot string i usually just delete them and move on.
as i am using mine for load development, and to target vel. for a boolit
hardness , or to adjust for my open sights
or if a load is having a problem grouping, i then use it for a diagnosis.
but if i am using like 4227 powder. i use it to try and predict
the powder charistics of hot vs cold powder.