I had a great load worked up in my 220 Swift using IMR4831.Holes were touching at 100 yards in previous groups.But went to the range yesterday and the group had opened up a bit and shot 1 inch lower.No wind and temperature difference from last range testing was about only 10 degrees.Anyone got any ideas as to what happened?And no the scope had not been messed with and the barrel was clean.

Ah that's it! You cleaned the barrel!

What is the bedding like.

Almost everything on earth is temperture sensitive.

Anyone have an independent lab test on powders or just advertising?

sll,
To answer your question in a word, yes. IMR powder are sensitive to temperature changes. I've seen it demonstrated many times. I doubt that ten degrees would make much of a difference though. In Alaska, I've seen a difference of 150 fps from summer, eighty degrees, to winter, -50 degrees. The Hodgdon extruded powders that carry the extreme label are advertised to not be sensitive. When I first heard of the claim, I didn't believe it. I used a 7mm 08 and H-Varget powder and ran my own test over a little over three years. New Mexico temperatures aren't as extreme as in Alaska, but they did run from 100 degrees to the low teens. The average variation caused by temperatures was slightly less than 20 fps. Now I use Hodgdon extreme powder exclusively. The test I made entailed four different lots of powder. Changing lots didn't seem to make a difference at all. The powder also burned cleanly, and due to smaller granules meters pretty good as well. I've also had the same results with Hodgdons' H-1000, H-4831SC, H-4350, H-4198 and Benchmark. I hope this is of some help. Good luck.

I WOULD ASK, WHERE DID YOU PUT THE AMMO ON THE WAY TO THE RANGE. IN THE CAR OVER NIGHT?? IN THE TRUNK AND IT WAS 90 OUTSIDE?? IT WILL MAKE A DIFFERENCE.

I can only presume the ammo you shot on the second outing was the same as the first.

Consider humidity, temperature, rest, your placement on the stock, lighting, and different lots of components (if you used a different batch of the same ammo).

If you used the same rest and components I would strongly look at how you placed yourself on the stock.

Keep good records on temperature, lighting, and humidity when you shoot. It will help you duplicate things.

Sometimes it is just plain hard to duplicate good groups on a different day.

Hope this helps.

Beau