Temperature sensitive powders?

[sawdustdad]

School me on temperature sensitivities...which powders/types, why, practical implications, etc.

I've been warned that some powders (700x, Clays) are temperature sensitive, and in cold weather the light loads used in CAS create some concerns.

What reference material would have this info?

[Unclenick]

Because powder makers keep their powder's exact characteristics proprietary, there is no such reference available. Indeed, if you look around the web even at something as basic as burn rate, you find different tables list the powders in different order but nobody has access to each other's actual data. You can trust each powder company to have the burn rate order of the powders they distribute correct, but not any other maker's powder.

So you are stuck with anecdotal evidence regarding temperature sensitivity, plus there are a few comparisons of data on Hodgdon's site in their description of their Extreme line of powders, which are intended to be temperature insensitive. I am not aware of any pistol powders made with temperature insensitivity in mind because the short ranges tolerate a lot of velocity variation before accuracy diminishes.

If you read this article by Denton Bramwell, you will discover that gun temperature is actually more important than powder temperature sensitivity in many instances. However, since most hunting shots are from a cold barrel, the powder performance will matter to you there.

There is a general equation for powder sensitivity for powders that are not specifically formulated to be temperature insensitive. It is included as an optional calculation in the QuickLOAD internal ballistics software. However, QuickLOAD does not include 700X in its database because the author says it has changed so many times over the years that he does not trust that any model of it he might include would continue to be valid for it over time. That also means, though, that your 700X may not have the same temperature sensitivity as someone else's did or does.

I can tell you the U.S. military tests ammunition down to -65°F. This generally consists of ball powders with magnum primers these days, so you can expect any military load to function in the cold for you, and that military primers and spherical powder or all the IMR stick powders will be fine with that arrangement. Nonetheless, velocities and exterior ballistics are affected by the cold (due to air density increase in the latter case).

[Jimro]

To add on to Unclenick's excellent advice, it really helps if you are more specific in what you are looking for.

If you are worried about light loads in cold temps for Cowboy Action Shooting because of low case fill and poor ignition, you can switch to a bulkier powder like Trail Boss or Red Dot and a magnum pistol primer (which requires another load workup). You might want to add more crimp to ensure positive ignition, or use a heavier bullet too.

Of course until you actually experience a problem, I wouldn't recommend changing anything. After all, you can't say much about a powders temperature sensitivity without the context of a given cartridge and load data.

Jimro

[sawdustdad]

OK, to be a little more specific. I am loading 2.8g of 700x under a 125g Missouri Bullet Co. Hi Tek coated RNFP or FPTC bullet in .38 special cases.

CCI or Win small pistol primers. good, solid roll crimp. Fired from either a Ruger Vaquero or a Winchester 73 clone lever action rifle.

Load chronos at 620 fps with a SD of about 25.

Concern is that in cold weather, could this load performance be reduced enough for the bullet to drop below 400 fps?

I guess I can go out this winter and see for myself, but I'm really just wondering what about the cold makes powder less energetic.

Uncle Nick's comments about cold barrel/dense air make total sense. I can see a cold barrel "robbing" some energy from the powder charge as it expands down the barrel, but I can't see, given the short duration, that this would have a material affect on muzzle velocity.

[F. Guffey]

I guess I can go out this winter and see for myself, but I'm really just wondering what about the cold makes powder less energetic.

sawdustdad, you did not ask me what I would do so I will make a suggestion. Place your ammo in the freezer for at least two days then remove and place the cold ammo in a cooler. Then haul the ammo to the range and fire it before the ammo has a chance to take on heat. This will allow you to make comparisons between ammo that is warm and ammo that is cold.

F. Guffey

many years ago the British decided to build a rifle to replace the 303. It was not to be, they refused to consider changing anything like the powder. After a few rounds it was not necessary to pull the trigger. The new design got so hot the rounds cooked off before they could pull the trigger. Seems it was about 1906 + a few years.

F. Guffey

[Hammerhead]

I don't think you're going to lose 220 fps in Virginia. I don't think you would lose that much even in the arctic.

I do a lot of cold weather shooting in Minnesota with handguns and have never experienced any noticeable loss of velocity with handgun powders.

OTOH 620 fps is cutting it fairly close in a carbine as far as a minimum safe velocity to get the bullets out of the barrel.

[sawdustdad]

OTOH 620 fps is cutting it fairly close in a carbine as far as a minimum safe velocity to get the bullets out of the barrel.

I should have clarified. The 620fps is out of the revolver. Same load is 850fps+ out of the rifle.

In CAS you need to meet a power factor of 60 (125g x 620fps)/1000 = 77.5.

Minimum velocity for this bullet would be about 500fps.

[Unclenick]

Originally Posted by sawdustdad

I can see a cold barrel "robbing" some energy from the powder charge as it expands down the barrel, but I can't see, given the short duration, that this would have a material affect on muzzle velocity.

If you read the article I linked to, the author did as Mr. Guffey suggests. It occurs to me the mechanism of velocity change may turn out to be changing coefficient of friction between bullet and bore with temperature. But I don't know for certain. Variation of coefficients of friction with temperature are alloy-specific and can be very squirrelly. I don't have data on gilding metal on either steel or stainless steel.

[sawdustdad]

Excellent article, Uncle Nick. Thanks for the link.

In the testing, it seems barrel temperature was a bigger factor than powder temperature, and as you've surmised, perhaps due to bullet/barrel friction changes. If that is the case, that effect would apply to any powder.

I'm surprised by both the extent of the change in pressure over the temperature ranges tested (nearly 20% from 55 to 95 degrees) and the linear nature of the effect. If you were to extrapolate this to 20 or 0 degrees (F), you'd potentially see pressures drop another 20%. (assuming linearity holds).

I'd see this as potentially something that would be a concern for hunters taking long range shots in very cold weather when they've sighted in their rifles in warm weather (or with a warm barrel.)

So many variables, so little data...

[AzShooter]

Best thing you can do is invest in a Chronograph. You can get one for under $100, like the Caldwell witch is very good, or a Chrony witch I've owned and had very good luck with.

Chrono on the hottest day you can. Heat effects you velocity more than cold does.

Chronoing is the only way to really know how your loads are acting.

[Jimro]

Concern is that in cold weather, could this load performance be reduced enough for the bullet to drop below 400 fps?

Not with a healthy crimp that keeps the bullet in place until there is positive ignition.

And in Virginia, I don't see you getting cold enough to shrink the bullet significantly to cause gas leakage and velocity loss from that route.

Jimro

[g.willikers]

From my own experience and observations, there might be enough variation in velocity due to weather temperature to just make a difference in making minimum velocity for competition purposes.

And it can happen, as one of the local hotshots discovered.
He used his regular and successful loads for a big important match located 500 miles north of us.
His loads failed to make major there on an unusually cool mid summer day.

f that's the concern, then the only way to be sure is to test your own loads out of your guns.

[sawdustdad]

Best thing you can do is invest in a Chronograph.

I do have a chronograph. That's how I know what my current load is doing. I will be doing some cold weather testing. Just trying to understand the physics of the issue. I can measure the result, but interested in the cause.

[Jimro]

Just trying to understand the physics of the issue. I can measure the result, but interested in the cause.

The physics devolve into multi-variable calculus really quickly.

The easiest way to visualize it is thinking that "cold" causes a slower burn rate, which means the bullet can get pushed farther into the bore than it would normally, and as the powder burns, the bullet gets further and further down the bore so the powder never has the conditions needed to build the normal pressure curve, it will be "shallower."

One of the reasons why a hot rifle matters more than hot ammunition is the friction increase between the bullet and bore because of an ever so slight expansion of the steel.

It's all fascinating, but it's not so exact a science that even the DOD can just slap a load together without extensive environmental testing.

Jimro

[jmr40]

Concern over velocity loss with handguns isn't as great as with rifles.

My research is geared more toward rifles, but common RIFLE powders will see velocity change anywhere from 1 or 2.5 fps for each degree of temperature change. The powders that are less sensitive still lose speed, just a lot less. Around 1/2 fps for each degree temperature changes.

It works both ways too and it does not have to be -30 to make a difference. If I develop a load at 70 degrees it could be 50-125 fps slower at +20 degrees. That wouldn't make any difference at normal ranges of say 200 yards or less. But on longer shots could. At -20 that load could be 90-225 fps slower. Using a powder less sensitive would only see about 25 fps slower at +20 and about 90 fps slower at -20.

If I develop a load during the winter at around +30 degrees it could be 70- 175 fps faster during the summer when it is 100 degrees. If the load I developed in the winter is close to a max load it could well be dangerous at 100 degrees. Using a less sensitive powder would only see around 35 fps difference.

How much of that relates to handguns I can't say for sure. But with shorter barrels and different powders I'd think the differences would be far less.

[SIGSHR]

40 years ago there was some interest in Blue Dot for 357 loads. One of the gun writers wrote he experienced very high pressures in freezing temperatures.

[Unclenick]

Bluedot evidently has some odd behavior. Alliant put out a warning not to use it with 125 grain bullets in .357 Magnum, nor in 41 Magnum at any load level. No further explanation, though.

In general, any combustible chemicals storing potential energy will require some amount of energy injected into them to initiate breakdown and release of that potential energy. Since the injected energy is heat energy in powder ignition, the colder a powder is, the more energy you have to inject to reach that threshold, and the more heat energy a cold grain adjacent to a burning grain must absorb to start burning itself. This is what normally slows the burn rates down as they get cold. There are some odd exceptions. IRRC, liquid nitroglycerin, when frozen, is more sensitive than when liquid at a nominal storage temperature. This is probably because it normally combusts by detonation and the solid form carries a shock wave more readily than the liquid form. But I don't know for sure about that detail.

From the above, I think it is fair to surmise that using a magnum primer will mitigate low temperature ignition sensitivity loss some. A magnum primer raises the pressure in the case, which will help energy transfer faster. Most magnum primers also throw hotter sparks. I would test the load with both magnum and standard primers at normal temperature. If you find a magnum primer that raises the velocity a bit. Make a note of how much and when temperature gets low enough for the regular load to be slower by that same difference, switch to the magnum primer that caused the increase.


Edit: Note that many chronographs are built from consumer grade electronics, many of which are only rated down to 0°C (32°F). Sometimes the chronograph will not only fail to function below the freezing point, but the screens can go black. You'll have to try yours to see. Battery failure to deliver adequate current for operation in the cold is another issue. Using Lithium batteries can help, as they aren't weakened by cold as much as standard batteries are.