Just got a new rifle (bolt- 6.5 Grendel-22” Howa barrel) Worked up to a mid weight load (25.7gr of IMI 8208 XBR) with 130gr sierra game changer OAL is at mag length (2.310”). Manual says max load is 26.4gr. I’m going up in 0.2gr increments, showing zero signs of pressure thus far. Shooting temps avg 25*F this time of year. Question is with these lower temps will I have to re-do the pressure testing in hot weather(that’s 90*F in these parts). IF I have to re test then what,in general, is a good amount to back off when I see pressure signs and still be safe in the warm temperatures? Put another way..I’ve heard once you see pressure signs it’s a good idea to back off 0.5gr, since I’m shooting in cooler weather should I back off more than that?
Load development in winter vs summer?
- Thread starter Kramerica
- Start date
The old guideline was for every 1F change the velocity will change by 1.5fps. Spring rolls around and temp is now 65F (increase of 40F) then expect velocity to increase by 60 fps.
That said, modern powders vary in their temperature sensitivity. Even with very stable powders it is better to be cautious and do a workup during your summer heat to test pressure and accuracy. If while developing a load you find a node where several different powder charges perform well you can always load to the lower powder charge knowing that as temp heats up the load will retain performance as velocity potentially increases.
The other thing is if you are in geography that has clear striation in temperatures berween summer and winter then it may make sense to have 2 different loads. I'm in Texas where it was mid 30's for a couple of days and then back to mid 70's. In Summer the heat can exceed 100F... I generally load for warm weather knowing that most of winter will range between 55F-75F (yes, one day running Heater and then next day flipping back on AC).
That said, modern powders vary in their temperature sensitivity. Even with very stable powders it is better to be cautious and do a workup during your summer heat to test pressure and accuracy. If while developing a load you find a node where several different powder charges perform well you can always load to the lower powder charge knowing that as temp heats up the load will retain performance as velocity potentially increases.
The other thing is if you are in geography that has clear striation in temperatures berween summer and winter then it may make sense to have 2 different loads. I'm in Texas where it was mid 30's for a couple of days and then back to mid 70's. In Summer the heat can exceed 100F... I generally load for warm weather knowing that most of winter will range between 55F-75F (yes, one day running Heater and then next day flipping back on AC).
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Ned Ludd
Silver $$ Contributor
The amount a given load will need to be backed off depends on a number of factors that include the temperature sensitivity of the specific powder used, the actual charge weight and internal volume of cartridge, and how close to MAX pressure the original load that was developed in colder temperatures was. It's probably better to think of such an amount in terms of a percentage of the total charge weight, rather than some specific number of grains. For example, if you have worked up a good load at safe operating pressure in cooler temps, a good place to start is usually to drop the charge weight by 1%, 2%, 3%, etc., at the higher temperature until the velocity is equal to what it was originally at the cooler temp.
The most critical problems in using a load worked up at cooler temps when it's really hot outside are safety issues including increased pressure and velocity, although the load may also show poor precision. Going the other direction in terms of temperature (i.e. high>>>low) is usually less an issue of safety, and more an issue of poor precision. In my hands, loads that were tuned at close to, if not quite, the very highest summer temps, let's say from around 80-85 degrees F, will usually still shoot reasonably well when the mercury hits 95 degrees, or even slightly higher. Strangely enough, those loads still seem to shoot reasonably well down into the 50s or upper 40s, whereas the loads I've worked up in the upper 40s to low 50s usually shoot quite poorly at 80+ degrees.
With a given cartridge and load, it's not too difficult to measure actual velocity for a few different charge weights during both cold and hot temps, to make a good estimate of how much the load will need to be changed. In fact, it is not uncommon to see a wide range of daily temperature extremes within a short amount of time either early in the spring, or late in the fall. That is when I typically try to get such testing done.
The most critical problems in using a load worked up at cooler temps when it's really hot outside are safety issues including increased pressure and velocity, although the load may also show poor precision. Going the other direction in terms of temperature (i.e. high>>>low) is usually less an issue of safety, and more an issue of poor precision. In my hands, loads that were tuned at close to, if not quite, the very highest summer temps, let's say from around 80-85 degrees F, will usually still shoot reasonably well when the mercury hits 95 degrees, or even slightly higher. Strangely enough, those loads still seem to shoot reasonably well down into the 50s or upper 40s, whereas the loads I've worked up in the upper 40s to low 50s usually shoot quite poorly at 80+ degrees.
With a given cartridge and load, it's not too difficult to measure actual velocity for a few different charge weights during both cold and hot temps, to make a good estimate of how much the load will need to be changed. In fact, it is not uncommon to see a wide range of daily temperature extremes within a short amount of time either early in the spring, or late in the fall. That is when I typically try to get such testing done.
Ned Ludd
Silver $$ Contributor
It's ok...certainly a place to start. In my hands, QL overestimates the effect of temperature on velocity with [relatively] temperature-insensitive powders such as Varget, H4895, H322, etc. That may be a conservative/safety thing, I don't know. I don't typically see nearly as much velocity change with those powders over a temperature range from the low 40s to upper 90s (F) as QL suggests there will be. I've looked at the QL values for how many fps velocity change is expected per degree F, and they definitely seem high to me. So QL may give you some starting information as a rough guide, but it won't beat actual velocity measurements at various lo/hi ambient temps.
Straightshooter1
Gold $$ Contributor
I agree, as I find my QL useful as a "rough" guide, though it does get me in the the ballpark. And with regards to temperature effects, I find it useful in comparing powders and seeing the effects for a change in one component, whether it's temperature, case volume, seating depth, etc. Besides being able to get a feel for the difference(s) from one powder to another, just getting inside the ballpark with QL helps reduce time and the cost of getting the load right.
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