Optimum Charge Weight and Temperature changes

[ratbuster]

In the past I worked up loads without laddering. I simply looked into what powder amounts are typically used by others for my application, trying various charges with 3 shot groups, followed by bullet seating depth trials beginning at touching the lands. Once I find the best seating depth I confirm with 5 or 10 shot groups.

I now want to try the ladder approach , hoping to refine what I have and obtain better vertical grouping. I realize that velocity is directly related to temperature, resulting in POI changes but once I have an OCW will it remain optimal as temps change significantly?

Thanks guys.

 

[mikecr]

once I have an OCW will it remain optimal as temps change significantly?

Maybe. What is the application for this shooting?

 

[ratbuster]

Maybe. What is the application for this shooting?

High volume varmint shooting, mostly Ground Squirrels. I really enjoy a gun that consistently groups .25" at 100 yards 80+% of the time. I get that from my Vartarg. OTOH my 20P does .3" maybe 25% of the time but it is not as consistent, averaging closer to .5 or so. An outlier for the 20P would be .75". I want it more consistent.

 

[Ned Ludd]

The answer is probably "No". No powder on the planet is completely resistant to temperature-induced changes in burn rate, although some are much better than others. Regardless of whether you are using OCW, a ladder method, OBT nodes, etc., when you identify/define the optimal charge weight window at a specific ambient temperature and then load to the center of it, you are essentially defining the upper/lower limits within which velocity/pressure isn't changing enough to change the load dynamics. In other words, you're loading to the center of a charge weight window (node) where velocity/pressure are minimally sensitive to external factors such as temperature. The temperature-sensitivity of a given powder is the major factor influencing the width of the window.

If you then shoot that same load at a different ambient temperature, the resulting change in the burn rate of the powder may also change velocity/pressure by enough to put you out of the node. How much the temp needs to change for that to happen will largely depend on the temperature-sensitivity of the particular powder used, and other factors influencing barrel harmonics. If we're talking about a temperature change of only 10-20 degrees F, many single-based powders can accommodate this. If it's going to be 30-40 degrees F (or more), it may be a problem. I am personally more concerned when the load is developed at, say about 70 degrees, and then I shoot it at 95-100 degrees, which is not uncommon in the summer. Not only can this cause the load to behave poorly, excessive pressure can also be a concern. Going the other direction, you have slightly farther to go (temperature-wise) before failed ignition might become a safety issue. In that case, your likely to primarily see a decrease in precision unless it's pretty cold.

If you can manage to find solid temperature-burn rate data for your powder of choice, it would be a simple matter to make small adjustments in the charge weight to keep the load very close to the original velocity. I have looked for such data for some time and have yet to find much that I would really trust. Quickload [reloading software] will let you do sort of a poor man's version of this, but I am not yet convinced as to how good its temperature predictive outputs are.

Another approach is simply to take as many velocity reading as you can at different ambient temps, so you can construct a table of your own over time that will allow you to make small adjustments to the charge weight so as to maintain the load within the velocity/pressure window you originally defined at a different temperature. It will require a bit of extra work, but may be very useful in that it will have been determined using your exact setup.

 

[jlow]

What gstaylorg has written is correct, but in terms of how much temperature will affect your load, apart from temperature sensitivity of the powder, another big factor is how wide is your barrel’s accuracy node? Some barrels are touchy because the nodes are narrow and in those cases, you are going to be more likely to be affected. Other barrels have such a wide accuracy node that temperature effects are minimal.

 

[Ned Ludd]

Yes, that is exactly what I was referring to with the line "other factors influencing barrel harmonics". Fortunately, those factors will remain fairly constant for a single rifle. Further, whatever those happen to be for that specific rifle, it should be very easy to define by the width of the charge weight window determined during testing. Once you know the width of the window, you can easily determine velocity at the two boundaries. Any change in ambient temperature sufficient to change velocity by half the width of the window (or more) will have the same effect on the load as would changing the charge weight by enough to achieve a similar velocity change during the original testing carried out within a specific narrow temperature range. For example, if your charge weight window is 0.3 gr wide and that corresponds to about 30 fps total velocity change, changes in ambient temperature enough to change the velocity by more than about 15 fps in either direction would also be enough to start moving the load out of the window.

 

[ratbuster]

I do deal with wide temperature swings. I typically develop loads in moderate temps but also in cool temps. I shoot varmints in temps ranging from very cool to hot (40 to 90 degrees). I have already begun to log velocities, charge weight and temperature for each of my guns. As I suspected, when temps are extreme I need to adjust the charge to bring velocities closer to the center of the accuracy node.

I really appreciate the insight guys.

 

[mikecr]

I use two guns for groundhogs during hay season,, that's all they're load developed(under same conditions) & used for.
I don't attempt to jump into colder seasonal uses because the bores would be different(even while cut rifled), but I believe it's possible as the others suggest.
Even during my limited use, ambient temps in the mountains vary 70-100deg. What I do to manage this is keep my ammo in my pants pockets(to normalize temps), and I carry an old tan colored tee shirt rag to shade my barrel/scope in the field.

I see people attempt to cool ammo/powder. But it's more field practical to heat it a bit, and load develop with it this way.
Seems easier to keep ammo warm in the winter, than cool in the summer. So keep it warm always.