Range Report MV and ammo temp for ELR

[Derek01]

For ELR shooting,what is the best way to analyze MV and ammo temp? Some say shoot and record MV throught the year but ATM conditions change too. Others say freeze the ammo and then take 5 rnds,temp scan it,shoot it and record MV, then repeat with 5 rnd groups of warmer and warmer ammo under the same conditions. Is the change linear? What would be an acceptable temp range for an accurate graph? I use hodgdons extreme powders to help keep the changes small. Thanks

 

[Aimsmall55]

Re: MV and ammo temp for ELR

Shoot and record. There are programs that take v/temp into consideration. I've never used them but I would assume your lot of powder would be the determining factor of how much change there is . That's why I buy in bulk. Just do the freezer test. I wouldn't worry about it to much if they ain't shooting back... Just my opinion

 

[Bob 964]

Re: MV and ammo temp for ELR

As temperature decreases, the air becomes more dense and your bullet should fly less efficiently and with a lower velocity. If you zero your weapon at 60 degrees F and fire it at 30 degrees F, the increased air density should cause the bullet to fly slower and have a lower POI on your target. If temperature goes up, impact goes up; if temperature goes down, impact goes down. Generally speaking, cold ammo doesn't perform as well as ammo at higher temperature. Higher temperature generates higher pressure, which generates higher velocity.

I remember seeing a formula that permits you to adjust for temperature.

Degrees +/- from 60F X Target Distance in Hundreds of Yards / 10

So... if the temperature is 90 degrees and I am shooting at 500 yards, I would subtract 60 from 90 and multiply this number (30) times 5, and then divide the result (150) by 10. The result is 15. So I would subtract 15 from 500 and adjust my scope as if I were shooting from 485 yards. If the temperature is 30 degrees and I am shooting at 500 yards, I would subtract 60 from 30 and multiply this number (-30) times 5, and then divide the result (-150) by 10. The result is -15. So I would add 15 from 500 and adjust my scope as if I were shooting from 515 yards.

If nothing else, it's an interesting happy hour discussion topic.

 

[jbailey]

Re: MV and ammo temp for ELR

Hey Derek-

Do you have a good chrono? Often times, that is your biggest source of error, more so than temp changes.

You certainly need to know the impact of air density on your external ballistics, but in terms of MV change, I would go out and shoot it now on cold day, record that data and then wait until a ball sweating day in August and shoot it again, same exact load (w/o too many more rounds thru the barrel). Make sure you use the same chrono in same lighting conditions (if it is a light chrono). Or you could heat up the ammo on the cold day and quickly get it into the gun and shoot it, so you have something of a cold / hot data points. As I said, chrono error might be a bigger source of error however...

 

[lanwickum]

Keep your DOPE as accurate and orderly as possible. Things are hardly ever, make that almost NEVER, the same when shooting ELR. This is what makes it fun. No simple answers shooting ELR. Even the position of the sun will change things a bit. At the same time, don't make excuses for shoot error. This is the #1 cause of POI shift from shoot to shoot.

 

[waste_knot]

I put 5 rounds in a baggy and put them in the in the cooler on ice. 5 rounds at ambient which was 80 deg. Chrono say about 0.7~0.8 fps per deg for IMR 4895

 

[MZ5]

Denton Bramwell has done a well-controlled examination of this type of question. His write-up can be had from:
http://www.shootingsoftware.com/ftp/Pressure Factors.pdf

Perhaps the most important take-away is that rifle (barrel) temperature is the most important factor; a much stronger variable than powder/ammo temp.

 

[LastShot300]

Here is another nice article, just a little bit complicated, on methods to compensate for Air Temp / Muzzle Vel

http://www.longrangehunting.com/articles/ballistics-temperature-velocity-1.php

 

[OneShotJim]

Below is a link to a good Sniper 101 video. Covers the importance of temperature both as an atmospheric factor (for air density) and the effect of powder temp on powder burn rate and thus muzzle velocity (MV). One's firing solution should account for both, especially at longer ranges. Note also that barrel temp/chamber temp can affect powder temp! - the longer one's cartridge is in the chamber the more it will heat up in a hot barrel. Here's the link FYI:

http://www.youtube.com/watch?v=uFrOwyRbDGA&list=PLJUaiRIEduNXoal2_PkBZi0vDCIcEPxUn&index=28

Shooter is one app that can account for powder temp. That's what I use.

 

[Derek01]

Thanks for the help. I also asked Bryan Litz the same question. His response was
Testing ammo for powder/MV sensitivity is difficult, as many variables affect the actual temp of the powder at ignition. Time in the warm/hot chamber for example. A good rule of thumb is 0.5 fps per degree for single based powders like the Hodgdons extreme powders, and 1.0 fps per degree for double based powders.

 

[timelinex]

For what it's worth, I chrono'd my 338 in the smear at 100 degrees and just recently at 50 degrees. It was with 5fps of the summer temperature. Keep in kind a couple things..1. I'm in the shade, so the ammo isn't getting heated by the sun. The only thing that's heating it is ambient temperature. So the effect won't be as drastic as direct sunlight.
2. Chronos are affected by different lighting conditions... No way to guarantee it was the same lighting conditions, so in reality it might be diff velocities slightly

 

[40xs]

I have historically used varget in MY f/tr .308 loads. I read on here where guys were getting higher vel. with reloader 17. So I got some and worked up some loads the last summer. Sure enough my velocity went up and my moa at 1k went down. BUT, in the winter months when I took same load to a match I was in the dirt and had to come up a full 4moa (40 inches). A big difference in point of impact from summer to winter. Varget would have been barely 1 moa difference. the danger is if you develop a load reaching max in the cold months and then shoot it in hot weather you will be way over pressure. Keep good records with temps. and adjust accordingly. I went back to using varget!!! ( wish I could find some!!) lol.... or cry in sadness!!

 

[Niles Coyote]

From a test I posted elsewhere....

Varget and temperature stability

From my Rifles Only class load, which was 43.2 Varget in New Winchester brass with 175 SMK’s

I got out to do some temp testing with my Varget load of 43.2 under a 175 in new Winchester brass which from my records for a 78-82 degree day was 2598 adv. for five. I set 5 cartridges outside overnight and temps were 32 degrees by testing time. That was not cold enough for me so I waited 4 hours while they cooled down in the deep freezer which is at 0 degrees. Adv. for five was 2540 one of which was 70fps slower than the rest so if I remove that from the sample the adv. changes to 2554, the ES for 5 was 78fps with the low one removed ES is 9fps.

So for an 80 degree difference 44fps to 58fps difference... or about .55 to .72 fps per degree.

Not Bad!

Also to note, once fired with the same load gains about 10-15 fps over the new brass average.

 

[LastShot300]

Nice and informative video. Thanks for posting it

 

[Niles Coyote]

Nice and informative video. Thanks for posting it

Just take it with a grain of salt until you prove it correct...