Odd happening at the range (cronograph)
I have had my chronograph for about 3 months now and like it. Till today the numbers it gave matched well with published data, but today it got weird.
I shot 3 strings for load development which tracked well with previous loads and published data. I then shot a known load which has always given good accuracy (1.25" groups) with an average velocity of 2665 fps. This load is in the middle of the load range and has never given pressure signs.
Today it did not give pressure signs but the velocities were very high. 2812, 2989, 3023, 2801, 2699 . As you can see all but the last shot is way far from the normal velocity. I did not remember the normal velocity so the first round did not bother me. But on the second I knew something was not right, all primers and case heads look normal and it was grouping just like always and recoil was normal, so I kept shooting them. I know I used the right powder as it was the same powder used for the other loads fired that day that tracked well. And all powder charges were dipped and then trickled up to weight so I do not see how I could have used to much powder, also the load is nearly compressed and I would have noticed during bullet seating if much more had been used. I do not think the chronograph gave good numbers.
The only thing I can think of to cause this, is that the rear of the chronograph was in the shade and the front in the sun, I was using the sky screens. Could this have caused this? Any thoughts?
I believe chronographs are supposed to be shaded if you are working in sunlight, for best accuracy. I don't know if this means both sunscreens should be in the shade, or both in the sunlight, but my guess is that would be the proper setup. Having one in the shade, and the other in the sun, might possibly cause problems. Chronographs are generally pretty accurate devices, but can be "fooled" by random reflections of light. The circuitry depends on light sensors that detect a change of light as the bullet passes overhead. If, for some reason, a reflection of sunlight reaches the 2nd sensor before the bullet actually passes over, then the chrono is fooled into thinking the bullet is faster than it actually is.
The other possibility is a chrono that isn't level to the bullet path. The chrono should be as parallel to the bullet path as possible. This is because if, for example, the bullet passes 1" over the first sensor, then passes 4" over the second sensor, the path traveled by the bullet is slightly different than what the chrono is seeing, and...in this example, will indicate an error reading on the high side. In your case though, I'd guess your machine got fooled by sunlight reflections bouncing in there before the shadow of the bullet arrived. Try the same load again on an overcast day...I'd bet the problem will be gone...
Speaking of accuracy...if you really want the highest degree...make an effort to keep all your shots at the same height above the chrono. One shot passing an inch over, and the next passing over at 5" really isn't being consistant, and might be one reason for a wide ES reading. (extreme spread). Try to use a solid rest, if one is available.
A couple of teething pains I've had with my crony and my learning curve:
Keep the crony the same distance from the muzzle, I use 8 feet with muzzle break rifles and 10 feet without.
Keep fresh batteries in your crony.
keep your chrony level with the bullet path. That insures that the sensors are at their proper spacing. Unleveled causes a faster indication kind of like shooting at an angle is a shorter distance.
I have a master chrony, I use the brass rings on the four metal rods that hold the white plastic shades as a bullet height marker. They're about 6 inches above the sensors and the cross hairs make it easy to adjust when using the brass collars for reference.
If your chrony is the fold up type like mine make sure it's unfolded completely flat. The tripod attachment can cause the back half to tilt up a bit. If that happens your sensors will be closer together and register a faster speed.
I put a cheap round bubble level in mine it works fine.
I agree that leveling the chrono is completely necessary, and the little bubble idea sounds great. While I was learning to use my Pact years ago it would regularly lie to me, like 1,600 fps with a 125gr out of a 2 1/2" .357! It took a while to realize it did not like bright sun and the light shades that came with it did little if anything to help. For years now I've been shooting in the shade, and when none is available I use a large cardboard sheet to provide shade. Now, if I can't shoot on a solid overcast day, or in the shade of a tree (equal levels of shade on both screens), or if it is too windy for my cardboard cover, I don't bother to waste ammo. I also have some "established test loads" that I use to confirm accurate readings. Varying levels of light can make even the best chronograph crazy, which of course makes the shooter feel crazy too. Folks in ballistic labs never even consider shooting out-of-doors, and have complete control over their "artificial" light indoors. GOOD LUCK WITH THE LIGHT!
Consider every possible variable
Was it a hot day? You will get higer pressures and velocities if the ammo temperature is higher than if it is cold.
As was stated before, if you are using a Chrony (or any other that folds up,) having the sensors tilted towards each other or partially folded will shorten the bullet path between detectors and yield an arroneous higher velocity.
Shooting at an angle across the chonograph (either vertically, as in not level or horizontally, as in not having the chronograph pointed straight downrange from shooting position to target) will give an erroneously lower velocity reading.
Muzzle blast, you probably already know, can give erroneous readings, and theoretically could be faster or slower than true. But your consistent readings (except for that last round) throw me.
Marshall, if the brass joints you use to set the height of your bullets' path above the chronograph are 4" above your Chrony, and your scope is 2.5" above your barrel axis, your bullet is only 1.5" above your Chrony.
"It's a mystery" (from the movie "Shakespeare in Love")
The center of my brass joints are 6.5" after seated in the Chrony. My scope is 1.65" from it's center to the barrel center. I'm shooting 4.9" above the sensors.
Originally Posted by Lost Sheep
2.5" is more in line with an AR style rifle. Based on his velocity of 2665fps I'm assuming he's shooting a 250gr 338WM or equivalent.