Velocity increase in cold weather???

S

Styled

Regular
EE Expired
Rating - 100%
9   0   0
Had something odd happen today that has me scratching my head....

I had been doing some load development in the summer at 20c, and today I was checking the same load to see how much velocity difference I would have at 0c so I could adjust the load. Everything remained the same including load components, distance to Chrony, Clean bore. Rifle is a Cooper 25-06AI with 26in Barrel.

Load: 25-06AI, Lapua 30-06 brass - FF - Neck turned, Reloader 22 (58.5grs), CCI BR2, Berger 115VLD's (.050 off). Every thing was consistent, same canister of R22, Ammo was exposed to the cold air, not heated in anyway.

58.5gr @+20deg - 3374fps Avg.
58.5gr @-0 deg - 3420fps Avg.
57.7gr @-0 deg - 3386fps Avg.

A drop of 20deg gave me an increase of 50+fps, if it was a drop of 50fps I would understand.. and be happy but the increase is weird.

I tested the Chrony with some other loads and its working fine.

Any ideas, or experiences?:confused:
 
That does kind of go against the typical temp/velocity changes. Be interesting if you had the same load sitting in the truck at 20 deg at the time and compared them to the cold loads. Who can say?
 
Was it sunny or cloudy teh days that you did this, it may be a chrony error.

Cold always decreases velocity to a certain degree, its a simple concept the cold weather reduces teh burn of teh powder and drops the PSI by a 1000 or so.

Unless your bullets were warm when you shot them. or sitting in a hot chamber, than you will get a increase with low density air, and warm or hot bullets.
 
The only thing I can think is that this is a slight change in your setup.

The bullet travel needs to be parallel to the chrony measuring plane to obtain 100% accurate results. If it is even 1 degree out it will give a slight error and if your setup is slightly different from one shooting session to the next your results will be different too.

50 fps is less than 1.5% which is not much of an error. I have wondered about how to go about setting up mine so that I can be 100% sure the shooting and chrony planes are aligned but figured it was just me being anal about it and have decided I can live with the miniscule error that my setup will incur...

Cheers, Bob
 
Could it be that the warm aid you originally tested in was more humid and thus the bullet had to push through more?

Have you shot enough to calcluate a standard deviation on your loads? This would tell you if a 50fps difference would be out of the norm for that load.

Could be chrony error - either outright or acceptable margin of error.
 
Thanks for the reply's guys.

Bottom line was that every thing should have been the same with the exception of the temprature and maybe humidity. I always try to set the Chrony at the same distance each time but as far as angle, or how close the bullet fly's to the censers these could have been different but not by much.

The other thing that crossed my mind was that I did crack a new bottle of R22 although it has the same lot number.

Not a huge deal i'll just have to adjust. Just irritating when you get a mystery across the Chrony that plays on your mind when your shooting for groups near/at/above max pressure.. :sniper:

Oh well the 57.7gr load at 3385fps is giving me .5 MOA to 300yrds, will just have to work up in summer again.. and start only buying powder in 5-8 pound jugs.
 
sunray said:
Reloader 22 is known for being temperature sensitive. Usually loses velocity though.
Click to expand...

REloder 22 is not temperature insensitive, that's why it usually loses velocity. I know in all the rifles I've loaded for that use re22 I would lose 100 fps from +20 to -10.
 
From what I understand is the less dense the air the less you need to worry about twist rate for stabilization, for example if you had a 1 to 9.25 twist for a 243 you can get away with shooting 105s in less dense air than at sea level, such as 90s or 95 grainers.
Air density will effect drag any way you look at it.

But I am no ballistic expert maybe Brian Litz wil see this post and tell us how it exactly works when shooting in different air desity
 
Such a small increase in vel could be due to many things.

thermal contraction of bullet/bore: Was the gun warm when you fired, or cold?

but only 50fps is well within the expected ES.
 
If shooting in high altitude the thin air would make a difference thus offsetting the colder Temperature. Havent' tried it but the two should cancel each other out....Maybe we have some sheep/goat hunters here that could elaborate ?
 
Well not to sure how dense the air was at the time I was firing but I haven't had a load "gain" velocity when the temperature drops.

The rifle went from cold to warm as I fired it, the average temperature of the barrel would have been cooler then when I fired it in the summer. I wouldn't care if the change of velocity was and average drop of 50fps then your right no big deal. But this is a 20 deg drop in temperature and an average increase of 50fps which means I have no confidence in firing this combination of components in the summer and will have to work my way up again.

I'll be testing again at the range tomorrow and see if anything has changed, it should be colder as well.
 
As far as I know most people set up their chrony less than 10yds out, so air density isn't a factor at all. Just to set the record straight, air density:
> decreases with increased altitude
> decreases with increased temperature
> decreases with increased humidity

Are you using an electronic scale? I have heard they can produce inconsistent readings with variations in temp.
 
You must log in or register to reply here.
Back
Top Bottom