Originally Posted by Buffalobob
My experience with those kind of tempratures is that things go bad quickly. A top end load will be OK up to about 85 degrees F and then pressures get high very quickly as temperatures increases. Three things cause problems as best I can determine from just casual shooting (no instruments to do scientific measurements).
First is if the cartridge itself is allowed to sit in the sun and get hot. I keep mine in a cooler and that is about 50 degrees F. Not cold and not hot.
Secondly, is if the barrel is subjected to both the ambient temperature of air and direct bright sunlight it will get hot on its own and not cool very well and will stay hot. Heat is then transfered to the cartridge and things get hot.
Thirdly is the firing rate may heat up the barrel and the inside will be hotter than the outside. I try to limit the time a cartridge stays in a hot or warm chamber to about 15 seconds on those days when I am too impatient to let the barrel cool.
Now you know why I advised you to reverify the rifle zero every day that you go and develop your drop chart. You might not know why things are different but at least you have a constant known zero. All of this work is what separates the men from the boys.
Thanks for your thoughts. I think in future I will just ditch shooting in these conditions unless I'm hunting and have too ?
I had to wait a long time between shots and also had to place the gun in shade to cool with a damp rag over the barrel. I'm sure the rifle was never dead cold at any stage though.
I think you are on the money because my first shot at 340 m was dead on ? So I guess as soon as this shot was fired, plus the extreme conditions changed things a little ?
So I take what your saying that due to the high temps, the pressures would have been increased greatly, thus causing the trajectory change ?
Also the ammo would have been warm from the direct sun.
Many thanks Bob !