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Old December 6, 2012, 05:09 PM   #11
Dan Newberry
Senior Member
 
Join Date: October 3, 2012
Location: Wytheville, VA
Posts: 215
We shot 100 match loads for .308 win last week, for a long range rifle course we host here in SW Virginia.

The 44.4 grain charge of W748 behind Nosler 168 grain CC bullets did fine. Velocity was a tad over 2700 fps from a 24" barrel, and this in 40 to 42 degree F temperatures. (and as an aside, the 1000 yard target was no problem for this load, turns out...)...

That load does not go 2850 fps in warm weather, I assure you.

The hypothetical and anecdotal data being tossed around the 'net is just that: hypothetical and anecdotal--most of the time.

If someone does a test to see what the velocity loss is from 100 degrees F to -20 degrees F, there is much more to be considered than we often see.

Was the velocity drop linear, respective to temperature? Or did it stay pretty stable all the way down to, say, 30 degrees and then drop hard? Or did it drop 80 fps from 100 degrees to 85, then stabilize all the way down the board?

Over the years, I've found that most information shared as to temperature sensitivity, or cleanness of burn, or lot-to-lot consistency is based not on personal experiences, but rather on what has been read elsewhere.

Powders can be more or less temperature sensitive based on load density as well. For instance, in a large temperature drop, you'll get more of a velocity loss with W748 on low density loads than on higher density loads, even though it's the same powder and same lot. There's a certain air to powder ratio in the case that's going to affect the overall pressure curve relative to temperature.

Also, proper load development cannot be over-emphasized. An OCW load (google "dan" and OCW) will show less temperature sensitivity than a non-optimal load. And by the way... just because your load shoots accurately most of the time, that doesn't mean that it's an OCW load.

Too, the "extreme" rated powders can have issues of their own. They tend to be harder to light, and therefore prefer high load densities which keeps a firm portion of the powder column pushed back against the flash hole. Extreme velocity spreads increase as load density goes down. However, there are other powders which have reputations for being sensitive to temperature than give fantastic ES numbers in lower, safer, and more comfortable to shoot densities.

So there's a lot to consider. I like W748, and I like IMR 4895. I prefer IMR 4350 to H, but use both.

The best thing to do is develop your load, and gather your hot and cold weather data in the field, rather than on the www.

Dan
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Last edited by Dan Newberry; December 6, 2012 at 05:17 PM.
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