Thread: Effects of cartridge temp on velocity

Before I start, it pays to note that the following dribble will not have much relevance to anyone that shoots normal hunting ranges and doesn't suffer OCD regarding, reloading.
Hopefully there may be something useful to some.


With the rifle calibers that I use, I have been trying to find a happy medium regarding temperature when doing load development. For accuracy at distance and most importantly safety.

My experience is that there is a small but notable difference in velocity of said loads between summer and winter.
This would be common knowledge to anyone into reloading and shooting.
Could be useful for those not aware and might be right on, or over a max load that has been developed during the winter months.

The velocity difference, varies slightly between the calibers I shoot. Mostly .223 and .260.

The .223 will differ on average 85fps on a common day between both seasons where as my .260 has a 60fps avg difference.

Reading a bit of guff on the subject the consensus thru some pretty stringent testing is that for every 1 degree change in Fahrenheit (1° C is 33.8 ° F) regarding cartridge temp, there is a gain or loss of 1.5 fps. This if right, would be dependant on a few variables.

So that's saying every 1°C the velocity should increase or decrease 50.7 fps?

So where I live the coast the average temp shift at sea level, between the seasons of say mid winter to mid summer, is around 15°C with an extreme day here and there.

In saying that, if cartridge temp is close to outside temp, a load doing 2950 fps on a 11°C day in the winter should increase by around 700fps on a summers day of 25°C to 3650 fps.!
That is if I have understood what I have read right.

Something has to be way off in those estimates, even with some other cartridge dynamics aside.

From my notes and in my rifle over the last few years the change between say 11 degrees and 25 degrees, the velocity difference observed is under 100 fps.

What I have applied from this is to do all my load development, or at least confirm all load development in the summer.

I then halve the difference in velocity that I have observed between seasons and have made a summer and winter profile on my ballistic calculator. I make sure I zero and input those atmospherics on an average day for those seasons.

This in turn has made a notable improvement in my expected POI when shooting at steel between 5 and 700. (As far as I have tested so far)

I used to get a 6-7 inch vertical differences on first shots and have till recently, put up with it.

Since applying my new data the vertical difference I have observed, so far, is now well under half that.












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Not quite Dave, 11 celsius = 51.8F 25 celsius = 77F so only a 25.2 F change x 1.5 = 37.8 fps change.

The celsius to fahrenheit conversion is not constant across the temperature range -40C and -40F are the same temp

Originally Posted by stug

Not quite Dave, 11 celsius = 51.8F 25 celsius = 77F so only a 25.2 F change x 1.5 = 37.8 fps change.

The celsius to fahrenheit conversion is not constant across the temperature range -40C and -40F are the same temp

Yeah uncle googled fucked me when I looked it up.
Thanks for that. Makes it all fall into line and realistic now.



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That's very good, intense, work @R93, with editing by @stug. The importance of observation and range notes, yes? And thorough research. Well done and thankyou. Will apply it to my own reloading. But not so sure 40C and 40f are equal! Might need to check again, Stug

okay, right, @stug, just observed the - . Didn't see it through the flyshit on my screen. Apologies.

It also depends on what powder you are using some are more temperature sensitive than others, some of the US long range shooters keep their ammo in a cooler bag/bin
to keep the temperature even through out the shoot, I have done it when shooting F class on a hot day don't know if it helped but it didn't hurt

Basically 9/5 conversion C to F, and 5/9 from F to C for degrees change.

So each degree C change will be 9/5 * 1.5 = 2.7fps/C

Originally Posted by shooternz

It also depends on what powder you are using some are more temperature sensitive than others, some of the US long range shooters keep their ammo in a cooler bag/bin
to keep the temperature even through out the shoot, I have done it when shooting F class on a hot day don't know if it helped but it didn't hurt

Thanks for mentioning that mate.

They were the next things I was going to explore. Powder and powder types.

And secondly if there was anything you could do too lessen the effect, by way of ammo storage on your person or in a piece of equipment like a cooler etc.

I have held a number of rounds in my pocket during the winter for a test.
Tried with both .223 and .260

Fired them over a v3 and the ES was all over the place compared to the same loaded batch of ammo I had on the bench.
Was an eye opener and sort of what led me down this path of thinking and testing.


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Greg Duley did a powder temperature comparison a few years back. Some powders were definitly worse than others.

Originally Posted by stug

Greg Duley did a powder temperature comparison a few years back. Some powders were definitly worse than others.

And yet some of those powders that have a bad name regarding temp, work really well for some over long term use and different batches.

RL 17 comes to mind. It was one of the most consistent I have used for one of my barrels but you could never get it when you wanted it

If I see more than a few bad reports on a powder I will generally avoid it tho.



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How about this, how does the air pressure effect the muzzle velocity? Does it have any effect. My first thought is that a lower air pressure (higher air temp) would give a higher muzzle velocity because there is less resistance in front of the projectile in the barrel. Or is it such a small factor the friction between projectile and barrel negate it?

Originally Posted by stug

How about this, how does the air pressure effect the muzzle velocity? Does it have any effect. My first thought is that a lower air pressure (higher air temp) would give a higher muzzle velocity because there is less resistance in front of the projectile in the barrel. Or is it such a small factor the friction between projectile and barrel negate it?

Not sure if it effects muzzle velocity too much but higher and lower pressures effect POI at distance.

You can see it on a ballistic calc with a 10 or 20 mb change.
So my dumb logic says the denser the air the faster the projectile loses velocity And vice versa?







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Yep agree with you there Dave about the effect of air pressure on the trajectory. Just wondering if some of the muzzle velocity increase at higher temp is also coming from the redcution in air pressure that comes with higher temp. Have you recorded pressure values along with temperature and muzzle velocity? @Norway might have an idea.

To convert temperatures in degrees Celsius to Fahrenheit, multiply by 1.8 (or 9/5) and add 32. (must add the 32)

Originally Posted by stug

Yep agree with you there Dave about the effect of air pressure on the trajectory. Just wondering if some of the muzzle velocity increase at higher temp is also coming from the redcution in air pressure that comes with higher temp. Have you recorded pressure values along with temperature and muzzle velocity? @Norway might have an idea.

Lately I have been recording everything.
But I have not got that info on me here in PNG.

In summer at home you can get a 27°day with 1028-1030mb
With the orographic lifting you can have a 27° day with 990-low 1000mb.

I have noted a difference for said range and what I have had to dial.
Just can't remember if it was much but there is a difference.

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