Loaded up a second batch of 308 Hornady 168 ELDMs and my bolt was hard to open. Same load data is used with the first batch in February (same powder charge, seating depth, primer and trim length). The fact everything was the same and that led me to think the temperature difference might be the culprit. In February the temperature was 35 and last week it was 105.

No powder is 100% immune to the effects of temperature, some are just a lot better than others. In some instances, a powder might appear to be very temperature tolerant when loaded in one cartridge but not so much when used in another.

How was your brass sized?

Did you also have extractor marks?

Did you have flow back around the firing pin strike?

Did you chronograph in February, and chronograph now?

In my experience H-Varget lives up to it's powder line name "extreme". It is extremely temperature stable.

Did you happen to open a new can of Varget. Also did the velocity go up.

Good question. Different powder lot can make the difference.

Was it the first round fired or did it happen later after the barrel heated up?
Did the round sit in the chamber for a while and heat up.
Was it a high humidity that day or a rainy day?
These can cause pressure.

Temp sensitivity typically shows up when loads are developed during cooler weather and then used wjen it warms up into the 90s or beyond. It can happen with just about any powder, "temp tolerant" or not.

Yup, been there done that.

^^^^ This, this, and this....

Also, sometimes on fired brass, after you reload it and shoot it, there is a slight difference between the virgin brass load and the fired brass load. I often have to drop my powder charge about .2 to .4 grains less powder on fired brass than the virgin brass load. It just depends on how you cleaned, sized, and loaded the ammo.

[/quote]Also, sometimes on fired brass, after you reload it and shoot it, there is a slight difference between the virgin brass load and the fired brass load. I often have to drop my powder charge about .2 to .4 grains less powder on fired brass than the virgin brass load. [/quote]

Chad, I assumed the opposite, I thought fired brass has a slight larger volume than virgin brass, hence using less powder on virgin brass. Could you explain?

If Chad wants to keep MV down to the virgin brass load, then that's the reason for dropping powder charge.

Virgin brass, energy is used to flow the brass to fill the chamber. With the exact same powder charge, in once fired brass, with a .001"-.002" shoulder bump, the exact same powder charge will often make more velocity. For the guy just loading for himself, that gain is a good thing.

I have done a head-to-head comparison and found virgin brass to produce lower velocity than multi-fired brass with the same powder charge. I presumed it was due to some energy used stretching the new brass to fill the chamber, as Fireman said.

The largest shoulder movement of my rifles is my 7 Rem Mag. I measured cartridge base to shoulder on virgin brass, then did it again on fired brass.

The shoulder flowed forward .014" and produced 50 fps more, using the same powder charge.

This is what lead me to one of my policies. Get all the brass you will ever want for the life of the barrel. For me, that is a 200 piece, minimum. Fire all of it once, size the shoulder -.001 to .003" trim, load, fire. Clean, anneal, size, trim, load. Keep this cycle until the primer pockets go.