badbrass said:
Thanks 280 ! What makes IMR so temp sensitive compared to H4350 when they sit next to each other on the burn chart? Sorry if this is a stupid question
is it just what's put in when it's made?
It is a good question, and has nothing to do with burn rates on the chart, but more to do with burn characteristics throughout a range of temperatures.
As you know temperature has an effect on many chemical reactions.
The Hogdon Extreme powders are more temp insensitive than most other powders, a trait they specifically manufacture for and advertise...which is a good thing (less velocity swing/POI change), I'm suspecting it's controlled by the coating on the powder (they are a greenish colour opposed to the standard graphite black/slate grey of the earlier non Extreme H powders and the IMR Powders of today (both the same company now BTW).
I've burned up 15 or 20 lbs of IMR4350, and while It shot well, I did notice a difference in the cold weather.
Anymore I run strictly H powders where possible (favorites are H4350, Varget, H4831).
Easy to check for yourself. Load up some H and IMR loads and chrony them on on a cool day (15 C). Keep some of those loads and leave them on the dash of your truck on a SMOKIN' hot day and chrony them. Finally this winter when the temps dip below -20C (I've shot down to almost -40C for wolves and coyotes) try them again.. .you'll see the difference guaranteed.
Will it make a difference? Not on a moose at 100yds.. but a good poke at 350+ on a deer (or coyote) it could mean the difference.
So with that in mind, I always use H Extreme powders to work up loads... one less problem that MAY occur...costs no more to take that extra precaution.
The worst powders I've played with for Temp sensitivity are RL 19 and RL 22, (and the IMR line) and I have not replaced any of them after I burned up the last few pounds a few years back.
RL 15 underwent a change a few years ago (apparntly due to a military contract and their Mil Specs), and is REPORTED to be much less sensitve now (I've not checked it), and when my last 3 lbs of RL 15 (old batch) are gone... I'll likely not bother either now that I have a good supply of Varget salted away.
Also REPORTED to be temperature insensitive is the Ramshot line of powders (a ball powder too - easy metering and supposedly very clean burning with a copper fouling inhibitor built in) from Western Powders. I have not shot any of these powders either but will give them a try one day when time allows.
280_ACKLEY
And not as much fun to carry afield 
