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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