When you use density altitude do you also record your temp. for your velocity variation?
Kestrel 4500 Shooters Weather Meter w/ Applied Ballistics
- Thread starter mildot
- Start date
When you use density altitude do you also record your temp. for your velocity variation?
I record velocity variation with temperature. But, I record it seperately since I'm not measuring velocity the majority of the time that I'm shooting.
I also do temperature tests on my ammo to get the variation with temperature ahead of time. Measuring it over different seasons is an inefficient way to quantify it...
I also do temperature tests on my ammo to get the variation with temperature ahead of time. Measuring it over different seasons is an inefficient way to quantify it...
Can you explain that a little further?
Typically I'll chrono every chance I get that way I have the best idea posible for the times I can't chrono.
The way the tests works is that you soak 3 batches of ammo at hot, cold and ambient temperature for at least two hours (5 rounds of each). I use a cooler with a heating pad for hot, the freezer in the clubhouse for cold (an icebox works too), and just sit the rounds out in the open for ambient. Temperature is measured with thermocouples and/or mercury thermometers.
You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to be the Jerry Miculek of loading a boltgun and firing it through the chrono. This is best done on a short pistol range, close to the berm.
You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to be the Jerry Miculek of loading a boltgun and firing it through the chrono. This is best done on a short pistol range, close to the berm.
A controlled test is the best way to determine temperature drift. You need to let ammo soak for a while before the internal temperature of the powder matches ambient temperature. Ammo brought from home in a plastic box will stay a the temperature it was in your home for a long time.
Some data I have sitting in my Photobucket:
Some data I have sitting in my Photobucket:
- Location
- Somewhere on the Hudson Bay Coast
I've used the older A-Trag version of the Kestral, and my only complaint was that the screen was difficult to see in bright daylight. The firing solution display is constantly changing with wind, temperature, and humidity input, which is kind of neat to watch for a while.
Deg. F
Kombayotch I'm very impressed with time / effort you are putting into the science of your shooting. Just out of curiosity, how much of a POI are we talking about at say a 300m target when comparing the cold vs hot ammo?
I've always read that Varget is not sensitive to temperature so I'm curious to see what that 50fps difference wI'll look like down range.
I've always read that Varget is not sensitive to temperature so I'm curious to see what that 50fps difference wI'll look like down range.
It doesn't actually take a lot of time. I start soaking the ammo when I get to the range, and fire it before I leave. The test only takes 15 min. to run, and that is because you're waiting a minute between each shot. People spend more time searching for temperature variation data on the net than it takes to run the test and it may be different for their load.
I don't shoot the ammo at a target when running the test, and you don't get that large of a temperature variation when shooting. But, it isn't as simple as a change in velocity. When the velocity changes due to temperature, it's no different that when you change the charge weight at one temperature. If velocity changes, pressure also changes (it must), so does barrel time (they are interdependent). So, the load gets re-tuned the same way it does when you vary the charge weight when working up a load. The POI changes not just from velocity change, but also from the barrel harmonics, the same way it would if you ran a ladder test.
Temperature stability is also important to maintain the precision of a load, not just for simplifying external ballistics.
I don't shoot the ammo at a target when running the test, and you don't get that large of a temperature variation when shooting. But, it isn't as simple as a change in velocity. When the velocity changes due to temperature, it's no different that when you change the charge weight at one temperature. If velocity changes, pressure also changes (it must), so does barrel time (they are interdependent). So, the load gets re-tuned the same way it does when you vary the charge weight when working up a load. The POI changes not just from velocity change, but also from the barrel harmonics, the same way it would if you ran a ladder test.
Temperature stability is also important to maintain the precision of a load, not just for simplifying external ballistics.
Run the numbers in your ballistic calc.
Just check the drop with your muzzle velocity and then add or subtract 50fps and see the difference. You may also want to change the air temp accordingly to get more realistic numbers.
