Retumbo & Temperature

[mt_archer]

I have been using Retumbo for years. It has proven to be decent for accuracy, and very good for stability and lot to lot variations. Recently, with a load for the 338 AX (Lapua Improved), I have had a great deal of variation (by Retumbo standards). Velocity was consistently in the 2975 fps range in cold weather (up to about 40), but since then, with warmer weather, has been consistently about 2900-2915 avg (in temps in the 60's and low 70's). Same lot of primers, same lot of powder, same lot of cases.

I have tried to make sure I have proper light conditions for my chronograph (CED MII). Trajectory indicates that it is probably reading correctly, even though I have not had the chance to shoot 1000 yards very much due to wind and rain. So, I have not totally ruled out my chronograph, but think it is probably accurate. In fact, this is the best chrono I have had for not being affected by sun angles and such, as long as I shade it from direct sun at extreme angles early and late.

An inverse temperature to velocity is not impossible, Li'l Gun has that problem well documented. But, has anyone experienced velocity going down as temperature goes up with Retumbo?

 

[Pdvdh]

An inverse temperature to velocity is not impossible, Li'l Gun has that problem well documented. But, has anyone experienced velocity going down as temperature goes up with Retumbo?

Yes, I have. But not to the extent of your 75 fps increase with decreasing temperature. Any increase at all with decreasing temperature was a novel experience for me. But I have repeated the experience with Retumbo more than once. And I run was running dual chronographs in tandem during these events, so I know I was receiving valid velocity data. I now run triplicate chronographs which provide three separate readings for each bullet fired. Well... I actually get 4 recorded velocities. My Oehler 35P proof channel provides the 4th velocity reading.

So yes I have seen it more than once with Retumbo, but more on the order of a 10-12 fps increase as temperatures dropped down towards 0 degrees F.

 

[Pdvdh]

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:

68F 2920fps
59F 2937fps
41F 2948fps
23F 2948fps
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.