Shooting over a chrono

[HeathSexton]

I jusy got a new chrono for Christmas and I get the 10'-15' from the muzzle, lighting, etc. How long do you guys wait between rounds? Do you treat it like load development and waita couple minutes between rounds or do you just run them through it? When putting my /06 through it today I got 2750, 2715, 2737fps with 180's on a cool barrel each round. When I fired with the barrel warm from a previous round I got 2699, 2686, 2690fps. SO the strings looked like this, 2750, 2699 - cool down, 2715, 2690, cool down, 2737, 2686. But that was today, I sent 3 across the other day 2670, 2670, 2661fps just bam, bam, bam. H4831 all from the same jub, 180NBT from the same box, WW brass from the same bag.


On a side note, I took it to the spot I can get out to 620yds and I was pretty consistent for me shooting. Once I got the drop and wind I went 8/10.


I shot my .243 +70SBK's over the chrono - 3410-3401-3405 - WHOA! It was awesome at 620yds.


Also, when shooting at long range, 300+, how long do you guys wait between rounds? I started shooting 2 rounds at 620 and then waiting around 5 mins for things to cool down???

 

[JD338]

When I set up my Oehler 35, its for load development so I usually have a few minutes in between shots.
This is the usual set up, here I am testing loads for a M700 LSS 7mm RUM.

JD338

 

[HeathSexton]

Also, what sort of ES should I be looking for? My .243 looks pretty good, the 30/06 gets hot quick and takes a while to cool, just going by feel the /06 in 2 rounds is warmer than the .243 in 2 rounds. I think the /06 would look better with more time.



Thanks for the answer JD, good looking shootin' iron.

 

[Mountain Goat]

I always make certain that my chrony is level, perfectly in line with my rifle and target, and I always try to shoot at the same height over the chronograph. There are too many other inconsitent factors that affect our shooting which we cannot control. Anything I can do to eliminated some that I can is well worth it to me.

 

[DrMike]

Heath,

I'm less concerned about the ES than I am about SD. A good rule of thumb is that a good load will not have a SD greater than 1% (e.g. 3000 +/- 30 fps). With good handloads (careful attention to detail), it would not be unusual that you would routinely witness SDs in single figures.

 

[FOTIS]

Single digit SD is what you want----ideally. :wink:

 

[HeathSexton]

Thanks guys, from looking at my numbers the SD from yesterday's 30/06 data my SD is 22. I went back and checked my notes, the brass was 4x fired and not annealed, I usually do every 3 firings, so that could have had some to do it with?

When I looked at the string from a few weeks back the SD was 5, it was on 1x fired brass though. I was a little shocked at the velocity difference with H4831, for it to be an Extreme Powder.

Velocity of 59.5grs in 15-18 degrees was 2668fps - SD - 5.

Yesterday it was 65 degrees and 59.5 showed flat primers, so I went down to 59grs - Average velocity was 2737fps SD of 22.


I also need to level up where I have my chrono, I am not level going through it but the distance is always consistent.

 

[DannoBoone]

What if one is shooting downhill (slightly) at the range? For example, our range has about a four
foot drop to targets. Would it be best to have the chronograph level, or better to have it in line
(parallel) with the travel of the bullet?

 

[SJB358]

Danno, you want it in line and parallel with the bullet for the best readings. Basically if you run a string through your barrel and pull it through the chrono is should run over the screens the same place. Scotty

 

[DrMike]

Shoot enough, and you'll eventually nail your chronograph. I shot off the ears on the left side of my Chrony yesterday. My first tip-off was when the velocity registered 4554 fps on a 185 grain VLD from a 300 WSM. That did seem somewhat fast. My second clue was the Chrony setting at an odd angle. This is the first time I've ever done that (though I have had friends shoot the ears off twice). I have had media or unburnt powder that nailed the electronics twice before. I've since learned to put an acrylic cover over the front of the Chrony. Fascinating stuff.

 

[257 Ackley]

:lol: Mike - I shot the legs off the right side of my chrony years ago and had to "Magiver" them for awhile. Welding rods fit into the slots just fine and have not been replaced yet :roll: .

 

[SJB358]

We overcome and adapt- I think I have heard that somewhere! Scotty

 

[DrMike]

I push the envelope for the number of targets I shoot off the bags. Normally, I'm pretty careful. However, I had a rifle that was a poor fit for me (thumbhole stock with a 20 inch barrel and a muzzle brake on a 300 WSM) topped with a Huskemaw scope. I could not get comfortable behind that rig, though I did manage to shoot a series of 0.75 inch groups with several loads. I was off by one millismidgen at one point and nailed the ears. I was really impressed that the Chrony kept on working very well sans ears. I do carry a spare with me, but I didn't need to pull it out.

Joel,

I have to ask how you managed to shoot off the legs. That is pretty impressive.

 

[257 Ackley]

Mike -
off to one side too much I guess. It took both the front and back legs holding the sky-screens off about an inch above where they slide into the box. I needed a quick fix and found some welding sticks in my brothers shop, which fit perfectly in the cheap chrony I have. They want almost as much for a new set of the legs and sky screens as a whole new unit, so I have just kept using the same setup for about 15 years.

 

[FOTIS]

Just to clear the air SD measures the uniformity of a load. What does this mean?
The load can be uniformly accurate or uniformly inaccurate. :wink:

 

[Guest]

To expound a little on SD, it is a measure of "variance" across the sample you're looking at. So in our case, it's a measure of the variance of velocity with a given rifle and a given load. Distilling it down with a couple of facts will make it pretty clear.

First, for a given load, approximately 68% of all rounds fired (of that same load in the same rifle) will be within a velocity range from one SD below to one SD above the mean velocity. Approximately 95% will be in the range of two SD's below to two SD's above the mean. That's why SD is important. A low SD means the load is very consistent.

Putting all this in an example so that it makes sense, consider that if you had a 270Win load with 130gr bullets, and the mean velocity was 3004fps, and the SD was 10, then 68% of shots fired would be 2994-3014fps, and 95% of shots would be 2984-3024fps. But, if the SD was 30, then 68% of shots would be 2974-3034fps, and 95% would be 2944-3064fps. That's enough to throw off both windage and elevation at longer ranges, and thus the importance of SD.