"Powder heat potential" for Retumbo???

[JimGnitecki]

There is a very useful Excel worksheet available as a download at:

http://www.accurateshooter.com/technical-articles/excel-formula-predicts-useful-barrel-life/

That worksheet attempts to predict barrel life based on load attributes, and is a nice tool, BUT it requires the "powder heat potential", in units of KJ/Kg, for each powder in order to be as accurate as possible.

It includes a table of values for common powders as a "comment" note attached to the cell that you insert this value into, but that table does not include "Retumbo".

The value for Retumbo cannot be "estimated" by looking at other table values, as H1000 is 3630 while 50BMG is 4010. That range of difference makes a huge difference in the predicted life of a barrel on this worksheet.

Anyone by slim chance have the value for Retumbo?

Jim G

 

[JimGnitecki]

Re: "Powder heat potential" for Retumbo???

A kind member of The Hide emailed me the number: 3710.

That led me to do an interesting comparison of different powders for both 338 Lapua (300g Sierra Matchking) and for 308 (175g Sierra Matchking), all using minimum recommended loads, to see what kind of barrel life the worksheet predicts. The results are, well, maybe not that believable, but here they are for others to chew on:

Powder / KJ/Kg / Grains / FPS / Pressure / Proj barrel life

338 Lapua starting loads:
Retumbo / 3710 / 85 / 2376 / 41,000 / 2459 rounds
H1000 / 3630 / 83 / 2383 / 43,200 / 2729
H4831 / 3870 / 71 / 2307 / 44,400 / 2635

308 Starting Loads:
Varget / 4040 / 42 / 2583 / 42,600 / 4364
IMR / 3890 / 41 / 2463 / 42,800 / 5508

The projected lives for both 338 Lapua and 308 look a little low to me, but then I am a novice at this precision rifle game.

But, if you believe the numbers, in 338 Lapua, H1000 is the most "efficient" in terms of fps obtained compared to barrel life.

In 308, the 2 powders are much closer, as Varget projects over 20% shorter barrel life but that's firing at 120 fps faster.

Interesting, but I;m not sure it will affect my thinking much.

Jim G