By John Kleespies
My car has a “hidden” storage compartment in the trunk, and I recently thought, “Hey, that’s a perfect place to transport my pistol and ammo.” I loaded up and headed to the range. When I got to my club, I had an “Uh-oh” moment when I found my 9mm ammunition almost too hot to touch.
Turns out, that little compartment sits right atop the muffler.
Oops.
I gave them a go, anyway, and the recoil was… snappier… than usual. As expected, I wasn’t concerned about shoot these particular rounds because they were my “bunny fart” competition loads: even a significant increase in pressure wouldn’t have pushed them over the edge of safety.
While some gun powders are marketed as being more “temperature stable” than others, they are all going to generate faster velocities if warmed to ambient temperature on an especially hot day, while conversely slowing velocities on particularly cold days.
How much of a difference, though?
There’s no simple answer to that question because every powder is different, as are the conditions in which they’ll be tested. That said, if we’re talking about a temperature difference that spans from tundra freezing to desert sweltering, expect an approximate 3% to 5% variation in velocity, just from the powder’s reaction to heat at the time of ignition.
In places where the seasonal temperatures differences can be extreme – think Alaska, and the like – the locals will oftentimes develop separate summer and winter loads for their guns.
To get a real answer to how much a particular load will sway in varying temperatures, then it will need to be tested at the various temperatures while using standard workup protocols… especially if you like to run your powders at max levels. Always remember that gunpowder is a fickle beast, and its anger grows at an exponential rate. If your load is sitting on the knife’s edge of “safe” on a cold day, then shooting it on a hot day could very well be the proverbial straw that breaks the camel’s back.
Scratching your head at having to test the same load in one weather extreme and then waiting six months to test it in the opposite weather extreme? Don’t fret! Just test on a hot day, chronographing one batch of rounds at ambient temperature… and refrigerate a second batch the night before, keeping it in a cooler till it’s transferred directly into the chamber.
What could also affect the powder temperature, beyond sunlight and car mufflers? Rattling off twenty-nine shots in fast succession – heating your gun to oven temperatures – and then leaving that last round chambered and baking for too long before pulling the trigger. That could happen even on a cold day.
And, of course, we’re only talking internal ballistics, thus far. If the ambient temperature is hot enough to cook your powder, then expect the surrounding atmosphere to be affected, too. Hot air is thin air, and thin air helps your bullet maintain its velocity over distance. If you’ve developed your load and zeroed it for shooting in the snow in January, then your dope sheet isn’t going to be worth much in July.
So, what does this mean in practical terms?
Let’s say that you test and dope a .308 Winchester hunting round in the spring or fall at an ambient temperature of 70 degrees, and you’re only loading to 95% of what would be considered a safe maximum powder volume. When you shoot that round in the wintertime at 40 degrees, then it may be 2% slower. Or, in the summertime, it might be 2% faster. In terms of where that +/-2% will cause the bullet to impact on a deer-sized vital zone diameter that’s placed anywhere from zero to 300-ish yards away…
Meh.
Now, let’s suppose that you’re doing your ammo development in the wintertime when it’s 40 degrees outside, and you’ve tested it up to a max powder volume, just shy of flattening the primers. In the summertime, if you decide to shoot that same load out to 800 yards on a 100-degree day without adjusting the zero or having previously work-up tested to verify its safety in that environment…
At best, you’ll miss the target altogether. At worst, you’ll need to explain to your wife why your face is all messed up, and you need to buy a new rifle. Either way, you’re gonna have an unpleasant surprise when you pull that trigger.
The moral of this story, then?
First off, I almost never load to maximum powder volume for this reason, amongst others. Think of it like this: if an unexpectedly hot ambient temperature pushes my velocity up by 3%, but if I halted my load development at 97 percent of maximum velocity, then I’ll still be in the safe zone if things go sideways at trigger time.
Secondly, if you’re a hunter or competition shooter, then it is always wise to zero your firearm in the area (and conditions) in which you’ll be hunting or competing. Simply grabbing a rifle and thinking that it will maintain the same point of impact at your home range versus in a different climate zone, well… that’s a recipe for disaster.
