There are times when I research data for a load development that I stay slightly lower than the max charge listed just to play it safe in case there is some quirk in weighing of the charge that produces more than I want and I might miss it. Until I can chronograph the load, I need an approximation of the velocity, and although I understand it’s not a mathematical truism, something is better than nothing.
For example, for a 120gr bullet in .25-06 Hodgdon lists Retumbo max at 60.0 grains and a velocity of 2991 fps. So I arbitrarily divide 2991 by 60.0 to get 49.85 fps per grain. I plan to use 59.0 grains, so I have an approximate velocity of 59.0 x 49.85 = 2941 fps.
I’m planning to use the following powders in .25-06 with a Speer 120gr BT bullet: (The relative burn rates are based on a list of 149 powders): Hybrid V100 (121/149); IMR 7828ssc (135/149); H-1000 (140/149); Retumbo (142/149).
I compared the starting charge with the maximum charge for each of those powders with data from Speer and Hodgdon, recognizing any differences can be explained by a difference in barrels and perhaps equipment they use.
I calculated the velocity per grain of charge as I did above, with each powder, expecting an increase in charge would deliver an increase in velocity per grain. But, not always. In some cases, both ends of the charge used deliver almost the same velocity per grain; in others, an increase, and, surprisingly, in some, a decrease in velocity per charge. For example:
Hodgdon: Retumbo 56.0 gr = 2806 fps = 50.1 fps/gr Max 60.0 gr = 2991 fps = 49.85 fps/gr
Hodgdon: Hybrid V100 46.0 gr = 2796 fps = 60.8 fps/gr Max 50.0 gr = 3009 fps = 60.2 fps/gr
Now, comparing Speer and Hodgdon’s data for the same powder:
Hodgdon: H-1000 52.0 gr = 2772 fps = 53.3 fps/gr Max: 55.5 gr = 2902 fps = 52.3 fps/gr (Decrease)
Speer: H-1000 51.0 gr = 2644 fps = 51.8 fps/gr Max 55.0 gr = 2922 fps = 53.1 fps/gr (Increase)
One might explain this by noting they did not use the same charge of powder in each case, but I wonder why Hodgdon’s data shows a decrease, while Speer showed an increase at the max charge weight-?
The same finding occurred comparing Hodgdon and Speer using IMR 7828SSC. Hodgdon, a decrease, Speer, an increase, in velocity/grain at the higher charge compared to the starting charge.
What might explain a lower velocity per grain of charge at a higher charge weight of powder?
This was a surprise: Speer’s data showed the same starting load of 51.0 gr with both H-1000 and IMR 7828SSC provided the same velocity of 2644 fps. This is despite a slower relative burning rate for H-1000. The max load was also the same at 55.0 gr, but the H-1000 velocity was 2922 fps while IMR 7828SSC was 2885 fps.- now, THAT can be explained by the difference in relative burn rate.
For example, for a 120gr bullet in .25-06 Hodgdon lists Retumbo max at 60.0 grains and a velocity of 2991 fps. So I arbitrarily divide 2991 by 60.0 to get 49.85 fps per grain. I plan to use 59.0 grains, so I have an approximate velocity of 59.0 x 49.85 = 2941 fps.
I’m planning to use the following powders in .25-06 with a Speer 120gr BT bullet: (The relative burn rates are based on a list of 149 powders): Hybrid V100 (121/149); IMR 7828ssc (135/149); H-1000 (140/149); Retumbo (142/149).
I compared the starting charge with the maximum charge for each of those powders with data from Speer and Hodgdon, recognizing any differences can be explained by a difference in barrels and perhaps equipment they use.
I calculated the velocity per grain of charge as I did above, with each powder, expecting an increase in charge would deliver an increase in velocity per grain. But, not always. In some cases, both ends of the charge used deliver almost the same velocity per grain; in others, an increase, and, surprisingly, in some, a decrease in velocity per charge. For example:
Hodgdon: Retumbo 56.0 gr = 2806 fps = 50.1 fps/gr Max 60.0 gr = 2991 fps = 49.85 fps/gr
Hodgdon: Hybrid V100 46.0 gr = 2796 fps = 60.8 fps/gr Max 50.0 gr = 3009 fps = 60.2 fps/gr
Now, comparing Speer and Hodgdon’s data for the same powder:
Hodgdon: H-1000 52.0 gr = 2772 fps = 53.3 fps/gr Max: 55.5 gr = 2902 fps = 52.3 fps/gr (Decrease)
Speer: H-1000 51.0 gr = 2644 fps = 51.8 fps/gr Max 55.0 gr = 2922 fps = 53.1 fps/gr (Increase)
One might explain this by noting they did not use the same charge of powder in each case, but I wonder why Hodgdon’s data shows a decrease, while Speer showed an increase at the max charge weight-?
The same finding occurred comparing Hodgdon and Speer using IMR 7828SSC. Hodgdon, a decrease, Speer, an increase, in velocity/grain at the higher charge compared to the starting charge.
What might explain a lower velocity per grain of charge at a higher charge weight of powder?
This was a surprise: Speer’s data showed the same starting load of 51.0 gr with both H-1000 and IMR 7828SSC provided the same velocity of 2644 fps. This is despite a slower relative burning rate for H-1000. The max load was also the same at 55.0 gr, but the H-1000 velocity was 2922 fps while IMR 7828SSC was 2885 fps.- now, THAT can be explained by the difference in relative burn rate.