Chronograph problems

[Silver_Rings]

I bought a Chronograph last fall to test my loads for my Chappy 1876 in 45-60.  I'll get a high or low reading (100 to 200 fps off the rest) sometimes in a 5 shot string that I just ignore.   

A couple of weeks ago I had a string where all the rounds were in the 300's, these are loads that are in the 1600's.  So I go out yesterday and the first 5 rounds are all in the 300 - 400 fps range for the 45-60, again these are loads that are in the 1600's.  I fired some 22 rf and they were in the 1200's which is about 100 fps faster than usual.  The Chronograph was set up in the sun with the screens, but the sun was hitting it at a fairly steep angle, doesn't account for the 22 rf reading about correct when the 45-60's don't.  The Chronograph was set up about 14 ft in front of the bench.  I moved it in to 10 ft where it was in the shade, the bullets passing over it were probably still in the sun, and tried it again.  Now the 45-60s clocked in the 1900's which is 300 fps faster than 2 weeks ago.  I'm at a loss as to what's going on.  Any ideas?

SR

[Ranch 13]

 Fresh battery, move it back a few feet, clean the screens. Mount a target to the front guide rods so you have a consistant aiming point.
Then stretch a plastic grocery sack over the top to difuse the light.

[larryo_1]

SR:
Don't know if this will help but here goes.  I have a PACT Professional series Chrono.  When I first started using it with my 45-75 I got goofy readings.  Called PACT and they told me to move the screens back to about 10 feet or so.  This is because of all that good smoke and stuff and also use it in the shade as direct sunlight can cause problems..  Done did that and guess what!  My results are as they should be.  I also found that having just a slight breeze helps too in keeping that smoke going the other way.  That BP smoke can raise holy hell if it is too close to the screens.  My maximum distance--as I was told--is 10 feet and it does just fine.  Hope this is of some help for you.

[Cimarron]

Occasionally I will have some odd readings with my Oehler 35P, usually in brignt sunny days.  The instruction book suggests that certain bullet shapes can cause a "glint" which in a given light condition (reflection from the ground) may cause the skyscreen not to see the bullet shadow or to misread it somehow.  This could be the reason your .22's bullets produced closer to normal readings than your .45's.  I am no expert by any means.  A chronograph is a great tool, when it is working     

[Silver_Rings]

Thanks for all the suggestions.  I'll give them a try.  I am shooting somkeless powder.

SR

[Trailrider]

As a long-time user of chrongraphs with optical screens, including the Oehler 35P and its "son" the M43 Personal Ballistic Lab, I can give you some suggestions:

First, I concur with moving the start screen to 10 feet, if your cables are that long.

Second, bright sunlight will give you more problems than a cloudy day, but sometimes we don't have any choice.  What I have found extremely effective is to use BROWN WRAPPING paper taped over the trapazoidal screen shades that are over the detectors.  Extend the paper tightly (not so tight as to warp the shades, but so the paper is straight), and extend the paper over the sides down to the detector boxes.  I use electrical tape to hold the paper temporarily, so I can remove and save the paper after use.  The brown paper should be translucent enough so that you can see some light through it when held up to a bright room light or the sun.  The stuff comes in rolls, and is relatively cheap.   

Third, with black powder and some slowburning smokeless powders, you need to keep the blast wave that exits the muzzle right behind the bullet from activating the start screen prematurely.  A piece of cardboard set up in front of the start screen, or a couple of feet in front of the muzzle, which ever seems to work best.  You can leave a small hole aligned with the screens to use as an aiming point.  The blast wave tends to spread out, so the cardboard should be at least as large as the shroud supports for the screens.

It sounds like a lot of work, but it will relieve a lot of frustrations getting good data.

Good luck!

[Silver_Rings]

Went out yesterday to try some of the suggestions.  I tape a white plastic bag up the sides, across the top and down the other side of the diffusers, new battery, and cleaned the sensors.  All shoots were from more than 10' from chrono.  Set the chrono up in the sun and tried it with my Schofield 45 Colt and speeds were about 620 fps.  I moved the chrono into shade and got about the same speeds.  Repeated with my Colt 1911 45 ACP and got 750 fps both in sun and in shade.

I then tried the 1876 in 45-60 in the sun and the first shot was in the 300's fps, the next shot was in the low 1200"s fps, next 1440 fps and third was low 1200's fps.  Moved the chrono to the shade and tried two more rounds and they were in low 1200's fps.  Took the diffusers off the chrono and tried a shot and it was in the low 1200's fps.  This is the same load that 4 weeks ago was in the 1600's and two weeks ago was in low 1900's.  

The one thing I did not do was put a sceen of some kind on the front of the chrono.  I'll give that a try the next time I get out.

Maybe the 1200 + fps is the real speed for this load in stead of 1600 or 1900 fps.  Makes me wonder how accurate my speeds were for my other loads I've clocked.

Silver Rings