Need Some Chrono Info

[bula]

Relatively new to having a chrono. A basic thing from Midway here now. A couple questions, what is the temp you want to chrono at, and adjust readings from other temps, to ? How many FPS per how many degrees to make the adjustment ? Just now, got velos of 1022 to 1054fps. It's 50 degrees out, partly sunny. A 5.5" S.S. 32mag.. Loading of 123gr PC'd HP, 5.5gr of HS-6, Starline brass and CCI primer. 12ft from chrono. School is open..!

[sharps4590]

35 years with a chrono and I never gave it a thought. I set up the chrono when the temperature was pleasant. Too cold, too hot, too windy, too wet, we both stayed inside.

[lar4570]

If you take notes for your load book, just jot down the temperature and date...
That way when you look back in a couple of years you'll have all the info you need.
I write group size in the margins, so I have more to go on than just velocity and standard deviation

[bula]

Found from a shotgunning source , possibly.. SAAMI uses 70 degrees for a temp base line for velocity, velocities ? Does that sound right ? If you shoot, chrono on a 50 degree day, how do you figure what your velos would be on a 70 degree day ? 

[ridenshoot]

www.americanhunter.org/content/how-much-does-temperature-effect-bullet-impact/

gundigest.com/more/how-to/firearm-training/ballistics-air-temperature-bullet-flight

[bigbrowndog]

I don’t worry about temperature variations affecting velocity unless you chrono on a day that is way out of your norm. For me that would be sub-50 degree days and days over 100. If I chrono on days like that then I will note it in my log and rechrono under more normal conditions. I do try and chrono under abnormal conditions if they are some I would expect to be using the gun under, like 100+ degree days for a long range gun used in summer time, or a hunting gun used in winter time. Some powders are affected more by temps than others, creating pressure variations, and these are good to know beforehand. I log all my chrono data on my ballistic program, and it allows atmospheric data to be added as well.

Trapr

[sharps4590]

Mar 1, 2024 16:50:01 GMT -5 ridenshoot said:

www.americanhunter.org/content/how-much-does-temperature-effect-bullet-impact/

gundigest.com/more/how-to/firearm-training/ballistics-air-temperature-bullet-flight

So, for a revolver with a 5.5 in. barrel, with a 123 gr. bullet, at a bit over 1,000 fps, it's insignificant and, though calculable, there's enough shooter error and differences in loads and individual cylinders to more than make up for any calculated differences?

[hunter01]

When shooting precision rifles, with certain powders, temp swings can take you out of your accuracy node if its to narrow. This can easily cause groups to double. Im not sure how it effects handguns or how broad their nodes may be. Im a new student to the revolver. I always found that if i chronoed in a mean temp, say 75-95*, it took some very serious temp swings to cause issue.

[ridenshoot]

I'm guessing there is no practical need to be concerned with temp swings and velocity unless the temperature affects the particular powder you are using, i.e.; very cold or very hot.  Since shooting handguns is usually occurring at relatively short distances there are other variables that would have a greater effect.  The accuracy of my powder scale is probably more likely to be a factor than the temperature.  Also, what is the accuracy of the chronograph that one owns, I'm thinking that for $150.00 my chrono isn't providing gilt edged accuracy and repeatability.

[bradshaw]

Mar 1, 2024 22:09:52 GMT -5 ridenshoot said:

I'm guessing there is no practical need to be concerned with temp swings and velocity unless the temperature affects the particular powder you are using, i.e.; very cold or very hot.  Since shooting handguns is usually occurring at relatively short distances there are other variables that would have a greater effect.  The accuracy of my powder scale is probably more likely to be a factor than the temperature.  Also, what is the accuracy of the chronograph that one owns, I'm thinking that for $150.00 my chrono isn't providing gilt edged accuracy and repeatability.

*****

I‘ve seen bizarre readings off some commonly encountered chronographs. In all the years I’ve used Oehler (old German-Texas name, pronounced “A-ler”) Model 33 and 35P chronographs, I experienced very few aberrant readings. The error pick-up is excellent. Typically, a pair of electric eyes see the shadow cast by a bullet: one shadow starts the clock, the second shadow stops the clock, Screen spacing must be precise. Variable light conditions can tax the ability of the equipment to differentiate between a bullet and an overhead cloud. A large muzzle blast, or a big, slow-moving bulet compressing the air in front of it, can cast a false shadow. A chronograph with its brains out in front of th muzzle invites problems.

One of Ken Oehler’s first tips is to set up a target, then line up the screens between the gun rest & target. There have been light conditions where I pass the bullet closer to the sensor (electric eye), or farther----just a matter of inches----to counteract light conditions.
David Bradshaw

[bigbore5]

I mostly try for accuracy over velocity. But velocity is an direct result of pressure. Once I find a good combo of powder and bullet, I will chronograph it and record it. When I get my next lot of the same type of powder, I will start with the usual reduced load and go back up until I am getting the same velocity that proved best before. This helps account for the variance between lots of powder.

[ridenshoot]

Mar 2, 2024 8:38:14 GMT -5 bigbore5 said:

I mostly try for accuracy over velocity. But velocity is an direct result of pressure. Once I find a good combo of powder and bullet, I will chronograph it and record it. When I get my next lot of the same type of powder, I will start with the usual reduced load and go back up until I am getting the same velocity that proved best before. This helps account for the variance between lots of powder.

Your point is well taken and that is why I tend to buy large lots of powder if it is possible. Multiple single pound cans or 4 lb or 8 lb jugs.

[bigbrowndog]

The light conditions issues David mentions, I’ve only encountered when I chrono in a direct sunlit condition. So I try and place my chrono in complete shade or chrono on overcast days when no direct sunlight lands on the chrono itself. The shade is relatively easy to accomplish by using either a large cardboard piece or golf umbrella or simply place the chrono under an overhang of a shed or shade of a structure. I have gotten errors when being too close to the chrono and using powerful rounds with large muzzle blast. I typically start at 10 feet away but have been moved to 15 foot on occasion. My ballistic program will extrapolate the corrected muzzle velocity when I input the distance to chrono, so it doesn’t matter what distance I will get a corrected muzzle velocity. My only issue is when I get a duplicate message on my chrono, (DUPL) I typically do not use that velocity recording in my findings. I just don’t trust it.

Trapr

[fm027]

Biggest variation in readings is caused by sun position. At least for me up north that's been the case. If I chronograph early morning or late in the day when the sunlight is coming down at the chronograph from either side rather than up above, you get incorrect inconsistent readings & lots more errors. Cloudy days negate that pretty good. But in full sun, try for when the sun is up high and nearest to verticle.

25 years of trial and error, shooting LR precision rifle where correct velocity means everything taught me this trick. At least with the old chrony's. Maybe the latest Garmin or labradar have addressed it somehow, don't know. Either way, keep the sun location and time of day in mind if you want the absolute most accurate and consistent numbers. It makes a difference.

[bradshaw]

Ken Oehler developed his Sky Screens----light diffusers----to create what he calls a “artificial cloud,” to neutralize direct and reflected light. I don’t use the diffusers when ambient light is too low for the sensors to see the shadow of a bullet passing above. Also, have removed the diffusers in a strong wind, which, by shaking the sensors, induces error.

All chronographs of my acquaintance measure INSTRUMENTAL VELOCITY; not muzzle velocity, which a nit-picker is free to calculate. Thus, the term "muzzle velocity” usually refers to instrumental velocity (velocity measured at the instrument).
David Bradshaw