Changes in velocity per grain of powder

C

cdoc42

New member
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.
 
Powder burn rates are not the same as in chamber performance.

It allows a kind of view of what area the powder performs in, but it does not predict velocity relatively or even non linear.

Add in powders that are on the high end of their go pump compared to another that is on the low end and you can easily have a 10% difference just in powder lots of the same stuff.

Ergo, start low and work high, not start around high. I start sometime at a bit below mid level.

Working with a 7.5 Swiss Barrel right now that has a wild reaction to H4831. Well off max and it still is not happy.

Have to sort out a powder issue, tight chamber or what.
 
Ah, it doesn't work that way cdoc42. I have yet to get the velocity stated in a reloading manual when running the exact same load listed over my chronograph. If you're really into math, knock yourself out with your calculations, but chronographs are relatively inexpensive and are the only true way to get velocity numbers.

Don
 
What everyone else is saying.

Wait until you go up .1 grains and the velocity drops lol. That used to confuse the heck out of me until either Bart or UncleNick explained it was due to the pressure expanding the case and was a warning sign I was on the ragged edge of a case rupture.
 
The calculation needs to be done a little differently. Divide the difference in the maximum and starting load velocities by the number of grains difference in the maximum and starting load powder charges. You will find this often different because the powder's relative burn rate isn't constant with pressure, and you generally want to know what's what up in the min-to-max range.

When you've done the calculation, what you have is an approximation of what would happen in Hodgdon's test barrel with the case and primer and bullet choices Hodgdon made. It can be significantly different from what you actually get, but it's a starting point. Here's how the pressure comes into play:

If your barrel is the same length as Hodgdon's and you get a lower velocity reading and you know your chronograph is accurate*, you can safely load up to Hodgdon's velocity and you will actually have a slightly lower peak pressure because, with a greater powder charge there is more gas made and therefore more acceleration is due to pressure nearer the muzzle, which will be higher with more powder.

If your barrel is the same length as Hodgdon's and you get a higher velocity than they did, then, even if you load down to their velocity, you will be at higher pressure than they are for the inverse of the above reason: less late-barrel pressure from the smaller charge.


*This generally means cross-checking with other chronographs or shooting over two at the same time for confirmation.
 
The other thing to watch for is free space. As you cross over from just less than full to compressed, there can be a change in how the powder ignition progresses. Do not expect linear changes in this region.
 
P Flados brings up a good point. Degree of case fill has an effect on powder ignition. There is an old axiom that says when using over 60 grains of powder in a rifle case, use a magnum primer.

Don
 
Unclenick said: "If your barrel is the same length as Hodgdon's and you get a lower velocity reading and you know your chronograph is accurate*, you can safely load up to Hodgdon's velocity and you will actually have a slightly lower peak pressure because, with a greater powder charge there is more gas made and therefore more acceleration is due to pressure nearer the muzzle, which will be higher with more powder.

If your barrel is the same length as Hodgdon's and you get a higher velocity than they did, then, even if you load down to their velocity, you will be at higher pressure than they are for the inverse of the above reason: less late-barrel pressure from the smaller charge."

Unless I misunderstand this, you're relating events that I might expect to see using the Hodgdon data in my rifle. I'm wondering why the results in the Hodgdon data don't show a consistent relationship between starting load and maximum load velocity changes.

If you get "X" fps per grain starting with 56.0 grain of powder, why didn't they get the same "X" per grain with 60.0 gr to account for the increase in velocity? Why did they get a higher velocity but a lower fps per grain contribution?
 
If your barrel's the same length but has a bigger bore and groove diameters than theirs, their same load will shoot slower in your barrel; faster if smaller.

SAAMI specs for the barrel bore and groove diameters typically have a .003" tolerance spread.
 
Before their was enlightment from the internet we were taught that when you were within the min and max for load data that you could expect to get about the same increase in velocity as the increase in powder, 10% gave you 10%, 5% gave you 5%. The Army also taught that if you went below the min data you could expect double increase in pressure for each percent you decreased powder. We never went below the min. That was when there wasn't a chronograph in ever pot and you learned to look at your primer for guidance. I see guys with $200 shooting mats and Ladaradars blowing primers and no clue even if there was a problem.
 
Powder is not so uniform as to have the exact same velocity every time if you weigh each charge. Some powder is very consistent even when thrown. You can go up or down .1 grain and find the velocity within the variation of the lower charge. Really It takes a .2 or .3 change to see a significant change in velocity for most pistol loads. It would be larger for rifle loads.
 
I'm wondering why the results in the Hodgdon data don't show a consistent relationship between starting load and maximum load velocity changes.
Click to expand...

I don't know with certainty, but it might be because modern powders are progressive.

They are formulated to burn under pressure, with the burn rate changing as the pressure in the case changes, until full burn and stability is reached. This might produce a series of steps, rather than a smooth linear progression, This would explain why you don't see a smooth consistent "fps per powder charge grain weight" through the entire range of powder charges listed in the load data.

There are lots and lots and lots of factors involved that simply are not in load data tables. These other things can result in real world non-linear results where just the math says otherwise.
 
Unclenick, what am I doing wrong? You said above,

"The calculation needs to be done a little differently. Divide the difference in the maximum and starting load velocities by the number of grains difference in the maximum and starting load powder charges. You will find this often is a smaller number than you get dividing the maximum velocity by the total charge because the powder's relative burn rate isn't constant with pressure, and you generally want to know what's what up in the min-to-max range."

Hodgdon lists the following for .380 Auto, 95gr SPR FMJ with CFE Pistol:

Start: 3.7 gr = 822 fps =222fps/gr Max = 4.2gr = 986 fps= 234fps/gr

Charge difference is 4.2 - 3.7 = 0.5gr
Velocity difference is 986fps - 822fps = 158 fps

158/0.5 = 316 fps -???? It is higher than the start and max velocity/gr-?
 
If you do a true line fit to velocity vs charge weight, it is not linear. People trick themselves into thinking it is when the equation works well over small increments, but still not linear.

Actually, there seem to be about 30 or so variables that Quickload uses. While it is pretty accurate, it too stubs it’s toe from time to time....
 
Cdoc42,

There are a number of reasons it doesn't work as simply as you hoped. One you are seeing is that the bullet doesn't start to move until the case is pressurized enough to release it and then some of the gas bypasses it and goes down the bore ahead of the bullet, wasting part of the charge. So you have to subtract that wasted powder from both the minimum and maximum charges before dividing them into the velocity, and because the pressures and pressure rate of rise don't match, it won't be the same amount in both instances. Another factor is that because the bullet barrel times are different and the pressures are different, the burning powder loses different amounts of heat to the bore. Another factor is overcoming bore friction uses a different portion of the powder energy in the minimum and maximum charges.

The bottom line is you have some differences that are rather a pain to quantify to allow you work from the total charge weights to find change in velocity per grain of charge change. The good news is that when you use the method I described, you've eliminated all the trimming considerations and just got down to what is happening after all that stuff is out of the way.
 
How hard the rifle is held against your shoulder also effects the muzzle velocity. I've seen a few dozen fps spread in average velocity across several people shooting the same rifle and ammo. And the extreme spread varied, too.
 
You must log in or register to reply here.
Back
Top