Re: Retumbo & Temperature
I just opened my ballistics software program to see if I had remembered correctly or not. Here's some data from my records - which is more accurate than my memory. The temperature versus muzzle velocity modeling for my 7mm Rem Mag with 74.6 grains Retumbo and the Berger 168 VLD:
77F = 3058fps
68F = 3054fps
59F = 3044fps
32F = 3039fps
14F = 3045fps
05F = 3044fps
Pretty temperature stable from 5F to 59F.
Here's the temperature versus MV data from my .280 RCBS 30-Degree Improved, loading 62.5 grains H1000, with the 162 grain Amax:
Approximately 28fps increase in MV as temperatures dropped from 68F to 41F with H1000.
With the .280 RCBS Improved, H1000, and the 168 grain Berger VLD, I see about a 5fps increase in MV as temperature decreases from 66F to 44F. Not as much of an increase as with the 162 grain Amax.
This is based on multiple chronographing sessions at a variety of temperatures. I'm providing averages based on sum of the chronograph data, as modeled by my ballistic software, LoadBase 3.0.
So I actually had a greater MV increase with decreasing temperatures with H1000 than I've had with Retumbo. But nothing close to 75fps. The increased muzzle velocities I've experienced are more like 5 to 25 fps.
Again, for the doubting Thomas's out there, this has been experienced over multiple chronographing sessions at a variety of temperatures from -5F to +75F, with between 2 to 4 chronographs recording each and every shot fired. And I have specifically allowed both the rife chamber, barrel, and ammo to cool down to ambient temperature prior to firing over the chronographs for the specific purpose of establishing muzzle velocity at varying ambient temperatures.
Whatever chemicals Hodgdon is applying to their Extreme line of gun powders, the result can be an increase in MV with decreasing temperature. But I have not experienced any velocity increases with decreasing temperature exceeding ~35fps. Never up to the 75fps increase mentioned by the OP.
Last edited by phorwath; 06-14-2011 at 11:50 PM.