TargetTalk

Hi Steve
The three chronos that tend to agree are within 4% ( as best can be determined +2.5 to - 1.5), the extreme error is about 7%.
Most of the testing was done with 10-15 strings of 6 shots.

The normal competition test procedure is to take 10 rounds at the start of the match, one is pulled and weighed.
After the match is shot, each competitor's ammo is shot from their own gun. If the first round passes the velocity X weight - end of test.
If it fails, a second shot is fired and the two shots averaged, if this fails then another is fired and the three averaged. If the average passes -OK, if not DSQ.

So far with the testing it appears that about 4% is the best you can expect in order of accuracy. More testing to come.

David:

Interesting- and here's something you might want to consider in developing a testing protocol that is fair to the competitors without being a real PItA.

The issue is you have to somehow deal with both "instrument error" (how far off the chrono might be from the "true" value of velocity) as well as the fact that there will be variance between each shot of a signle load formula. So what you want to do is make sure the average measured velocity from among a series of shots, as measured by more than one chrono, is above a certain threshold value.

Let's say you shoot a five shot string simultaneously through 3 chronos lined up in series. You could just take the "grand average" and "grand standard deviation" and then make the rule that "your average velocity must be 1.5 standard deviations above the minimum."

Couple of other ways to do it of course. But you want to balance the need to not unfairly disqualify an honest shooter by mistake- OR let a cheater sneak through either!

Steve Swartz

(you probably aslo want to weigh more than one head for your mass measurement . . . average weight of say three bullets although bullet weights are pretty darned uniform for the most part)

Steve,

Our Ballistic Research Facility has noted an anomaly with most chronographs, even the "Big Daddy" expensive ones, when measuring projectiles at or near the sound barrier. As best as we can tell, decelerating through mach near the screens creates a false trigger, either from the pressure wave or shadow (or both) near the actual projectile as it passes through the screens.

Our high tech solution is to set up a cardboard barrier just before the first screen. Using the first (unrecorded) shot as a guide, a small square is cut for the remaining rounds to pass through. The cardboard blocks the pressure wave and eliminates the false readings. Supersonic projectiles work just fine without the cardboard barrier.

Your tax dollars at work.

Keith

Hi Steve
The bullet weight, if a competitor thinks that his weight is light, he can request that another bullet be pulled and weighed.

My thinking so far for a suggested rule change for testing would be :-

1/ Prior to a competition a series of shots if fired across two chrono's in line, if the results are within a 4% range. then you use the highest reading chrono for the match (the other is a spare incase it gets shot or breaks).

2/ If not within 4% a third chrono is needed (inline) and another series is shot. You then use the two that tend to agree (within 4%) and take the higher reading one as the match chrono.

3/ if all exceed 4% then no chrono is to be used for the comp.

I have mentioned it to Spencer, but lots more work is required.