Chrono Accuracy - Do Distance and Bullet Path Affect it?

[RaylanGivens]

I have a ProChrono Digital chronograph. Like everyone, I'm trying to get the most accurate bullet velocity measurement of my hand loaded ammunition. I shoot in USPSA's Production division.

Is the distance from the end of the gun barrel to the chronograph critical? It seems like lots of people walk up to a Chrono and estimate that they are 10 feet from the unit and start shooting (controlled shooting under an RO of course). Does it make any difference between shooting at 8, 10 or 12 feet from the chrono? Is it worthwhile to measure 10 feet with a tape measure?

Does the path of the bullet through the chronograph make a difference? I usually shoot towards the top of the two arches on a ProChrono Digital. I do that to avoid hitting the chrono, but recently got to thinking that shooting high might be affecting the accuracy of the chrono. Is it more accurate to shoot right down the center of the chrono's tunnel?

[chuckols]

The 10' rule between the muzzle of the firearm and the Chron is a means that avoids affecting the light intensity aeen by the Chrono mirrors. Muzzle blast(light source) is the issue. Too close and your data may be badly distorted as the mirrors vision can be erroneously affected by the light input from the blast. The mirrors on each end are used to measure the time between entry/exit of the passing bullet. If you shoot really high over the Chron you run the risk of best detection of the passing projectile.

If you want your data to be internally consistent(relatable from shoot to shoot) then you should try to always do things in the same manner-take a tape and measure it if you wish is one example. Shoot in the center, use the same distances for setup and try to always provide good ambient lighting for tests.

Light quality(source) is critical to good Chron performance. Sunlight is the best source of full spectrum light but avoid glare. Shade the unit so it has an uninterrupted ambient light source. When you operate indoors then incandescent is the best with fluorescent a NO No!! The flicker rater is a disturbance to the mirror reading. If you have overhead can lights indoors then set the unit so the mirror does not look AT the source but sees only a diffused empty lighted objective area. I put a piece of paper over the forks (canopy).

Remember, the Chrono is measuring the time that the projectile takes to pass over the inlet and outlet mirrors, The bullet can be seen by the mirrors as a change in light intensity(shadow?) and the time delta is the input for the software to calculate the bullet velocity and all the other stats the system provides for a string of identical rounds being tested against one-another to measure the volatility of their velocity.

Edited January 7, 2014 by chuckols

[RePete]

Measure the 10' distance (or cut a 10" length of 550 cord). This is the distance for USPSA/IDPA/IPSC.

You might want to look at this thread to.

http://www.brianenos.com/forums/index.php?showtopic=186285

[Sarge]

I measure 10' every time I set mine up. I don't think it matters how low or high you are withing the screens. However, I did get some fluctuations by shooting on upwards or downwards angles through the screens.

[Steve RA]

Put some tape around the uprights 1/2 way up, try to consistently shoot at that level.

[ZackJones]

Put some tape around the uprights 1/2 way up, try to consistently shoot at that level.

That's a great tip! Going to tape now before I forget it

[ROSW]

Other than the bullet being detected by both the front and back sensors, the next most important thing to accurate readings is to have the bullet go directly through the two sensors (the shortest distance). Any angle will increase the distance the bullet must travel which in turn decrease the reported velocity.

[9146gt]

Try this with the chrono. I always shoot over the chrono at 10 feet in 20 rd strings. Use the chrono session as shooting for groups practice. With the chrono at 10 ft pick your target distange ( I usually shoot 25yds).Shoot 20 rounds with the target at the top of the screens then middle and botton and compare. After trying this with CED and Oehler it seems like you get higher FPS the lower you shoot in the screen. I now put the target in the center of the screens.

If you really want to confuse yourself,try shooting the same load over the chrono two handed, strong hand, week hand. It is real easy for me to shoot 100rds in a chrono sesson .

Tom

[RaylanGivens]

Great answers... Very helpful... Thanks to everyone that replied... I will measure the 10 feet every time... It makes sense to do that, plus the time and dollar expense is minimal... $12 tape measure at Home Depot...

Imteresting comments about Chrono light sourse... I have access to outside shooting bays that are covered... That should keep glaring sunlight from shining directly on the Chrono...

Makes sense to shoot a straight path through the Chrono unit... At the speeds these bullets are going, a small angle might make quite a bit of difference...

As far as shooting further down under the screens to increase FPS... I think I'll start shooting in the middle and just consider any possible FPS loss to be an extra safety cushion... Shooting in the middle makes sense... Liked the tip about putting tape around the center of the bars that hold up the top... Great idea... I'll do that...

[Sarge]

Put a target about 20-25 yards down range of the chrono right in the middle of the screens. Gives you a good aiming point and gives you a good idea idea of the accuracy of a given load. Saves ammo and trips to the range. A round that makes PF is not worth diddly if it is shooting 8" groups.