Gunpowder Temperature Sensitivity: Fact or Fiction

It was hot, July-in-Utah hot. I chambered one of the special handloads I'd developed six months earlier and shot at my 400-yard gong. I missed. Worse, when I opened my action, smoke poured out of it and a loose, soot-blackened primer fell out of the base of the empty cartridge case.

I'd developed that handload in 20-degrees Fahrenheit. Clearly, the heat had caused a pressure spike.

Heat can not only turn apparently safe handloads into dangerously hot handloads, but — legend has it — it also affects velocities and can cause shots to go awry.

Some gunpowder has the reputation of being particularly susceptible to variations in temperature. Alliant's Reloder line is one, for example. Others, such as Hodgdon's Extreme line, are purposely designed to be stable in extremes of heat and cold.

Trouble is, many propellants with a reputation for instability in extreme temperatures are very good powders that make assembling accurate hand loads easy. Reloder 22, for instance, is my all-time favorite powder for magnum rifle cartridges because it is quite easy to get it to shoot well.

The question we're tackling here is threefold:

1. Is gunpowder sensitive to temperature swings?

2. Are some propellants more temperature sensitive than others?

3. If so, are the velocity variations caused by extremes in temperature enough to change point of impact at hunting ranges and cause a miss on an animal?

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First, let's establish what "hunting ranges" means. Most hunters have no business shooting at game past 300 yards, but we'll give ourselves the benefit of the doubt and make 400 yards the outside parameter of our standard, common, hunting-range category.

But what about all the long-range shooters out there now that consider connecting on a deer at very long distances to be the greatest "trophy" of the hunt? To them, 400 yards is close. So if temperature-induced velocity variation does indeed occur, let's also examine the effect on point of impact at 800 yards — double our typical "long" distance.

To test whether temperature-induced velocity anomalies are fact or fiction, I loaded 10 rounds each for the 7mm Remington Magnum, a popular long-range hunting cartridge. I used Nosler's 160-grain AccuBond and two propellants, Hodgdon's H-1000 — which has a reputation for stability in temperature extremes — and my old standby, Reloder 22 — which has a reputation for volatility in temperature extremes.

Each batch was loaded with charge weights near maximum but previously proved to be safe in that particular rifle.

Half of each batch I put in the freezer overnight. The next morning, I set the ammo box on the dash of my truck and let it sit in the August sun until after lunch, by which point the interior of my pickup was stiflingly hot. Thus prepared, I loaded up my chronograph and rifle, put the frozen ammunition in a small cooler with several ice packs and headed to the range.

Ideally, I'd have frozen the rifle between each cold-round shot, but, unfortunately, the range doesn't have a deep freeze, and I wouldn't have had time if it had. To minimize the effect of a warm chamber, I attempted to send each shot through the chronograph screens within three to five seconds after dunking the cartridge into the action. I also allowed the rifle to cool — as much as it would on a 95-degree day — between five-shot strings.

With frozen-ammo velocities in the bag, I then shot the ammo still cooking on the dashboard of my pickup. The cartridges were hot to the point of being uncomfortable to hold.

The result? The answer to the first of our threefold question is yes: Temperature extremes do affect velocity. Both propellant types shot significantly higher velocities with the hot ammunition.

The answer to the second question is also yes: The disparity was much less with one propellant brand/type than with the other. Specifically, H-1000 had 44 fps disparity between temperature extremes; RL-22 had 130 fps disparity. The difference between propellant types — 86 fps — was significant.

Here's a breakdown of the results with each gunpowder type:

Hodgdon H-1000 - Frozen - Hot - Difference

Velocity (fps): 2,892, 44

Extreme Spread: 70, 12

Standard Deviation: 25, 5

Alliant RL-22 -  Frozen -  Hot - Difference

Velocity (fps): 2,972, 3,102, 130

Extreme Spread: 34, 6

Standard Deviation: 14, 2

As an aside, an effect I didn't foresee was that hot ammunition — at least in this case — is more consistent than cold ammo. Take note of the far tighter extreme spreads and standard deviations.

To answer the last of our threefold question (is temperature-induced variation enough to cause a miss at hunting ranges) we need to run some ballistic calculations. Keeping things very simple, I used the calculator on Hornady's website and plugged in the appropriate numbers. Here's what I found:

With RL-22 powder and a 200-yard zero, the 160-grain Nosler AccuBond (B.C. .531) will drop 18.8 inches at 400 yards with the frozen ammo and 17.1 inches with the hot ammo. The difference is less than two inches. With H-1000 the difference is less than one inch. So the answer is no: When using a temperature-sensitive propellant, even extreme swings in temperature won't cause enough disparity in point of impact to make us miss a game animal.

How about for long-range shooters that really stretch the distance? Let's double the distance and run the numbers:

Again with RL-22, the projectile drops 144.5 inches with the frozen ammo and 131.2 inches with the hot ammo. The difference is 13.3 inches, which is certainly enough to cause a complete miss on a deer-size animal, and a miss — or worse, a wound — on an elk-size animal. Clearly, as distances stretch, temperature sensitivity becomes far more critical.

Let's look at H-1000: At 800 yards, the bullet drops 153.8 inches with the frozen ammo and 148.6 inches with the hot ammo. The difference is much less, only 5.2 inches. Depending on the size of the game, that's still enough to potentially cause a miss, but the disparity is more acceptable.

We don't have space to address it here, but another element that exerts additional drag and drop on cold ammunition is the density of colder air. Lower temperatures result in more drag on your bullet, which will open up the disparities shown above even more.

The multifaceted nature of this "fact or fiction" topic bars a simple yes or no answer, but we can come away with this:

Yes, temperature does affect velocity, and it affects some propellants more than others.

However, within common hunting distances out to 400 yards or so, the disparities are little enough that we don't need to worry about them.

If you shoot long range, though, you'd better be familiar with your velocities and the trajectory of your bullet in the conditions in which you'll likely shoot. If you hunt in very cold weather, get out and practice in very cold weather. Leave your rifle and ammunition out to freeze overnight, and chronograph your ammunition. Then plug those numbers into your ballistic app.