How Temperature Sensitive is Black Powder?

[max]

I've heard and even experienced reduced sensitivity with smokeless powder loads in very cold weather and was wondering if blackpowder suffered from the same malady.

[Daryl]

Powder temperature has the effect of reducing the velocities and sensitivity of black powder, just as it does with smokeless, but to a lesser extent, I believe.  BP is the easiest powder to ignite at any temperature (lowest ignition temp), hence it's explosive classification compared to smokeless or the subs. What's fun, in a macabre sort of way, is reading some of the in line threads at other forums during the winter or summer when they are having trouble getting their stainless and plastic guns to go off.

[Mad Monk]

I've heard and even experienced reduced sensitivity with smokeless powder loads in very cold weather and was wondering if blackpowder suffered from the same malady.

The thing with black powder is that you must first "pump" heat into it in order to get heat out of it.
To support powder combustion you must first raise the temperature to the melting point of the potassium nitrate and then continue heating until you reach the decomposition temperature where it will then release oxygen to support additional combustion.  When you first ignite the powder you are starting this in localized spots on the very surfaces of the grains.  Once the spot, or spots, begin to burn and give heat the whole process spreads and speeds up.  So it will take more calories of heat to get a charge going at 20 degrees than one at 90 to 100 degrees ambient temperatures.  So the slowing of the ignition and initial burning of the charge will drop velocities a bit in the gun.

The ease with which black powder ignites and burns is why it is still used in large-caliber artillery as an intermediate primer to spped up the ignition of the main charge of smokeless.  Also used as an imtermediate igniter in aircract cannon rounds where high-altitudes at low temperatures would make the smokeless charge almost impossible to ignite.

Bill K.

[The other DWS]

Bill, I usually associate low temps with low humidity, would lower humidity have much effect?

what about last minute priming with a priming horn kept under your jacket where it'd be nominally warmer?

or are we talking about such relatively little difference that it would have little impact in hunting?

[Mad Monk]

Bill, I usually associate low temps with low humidity, would lower humidity have much effect?

what about last minute priming with a priming horn kept under your jacket where it'd be nominally warmer?

or are we talking about such relatively little difference that it would have little impact in hunting?

With the priming powder you would see no difference.
The brands of BP we have today are fairly moisture resistant so humidity is no big deal.  With any of the black powders today.  When exposed to air for any length of time they will reach an equilibrium with the air as far as moisture goes.  You don't see any measurable burn rate depression with moisture pick up, by the powder, until you get up around 1% or higher.  If you expose the powder to air at 50% relative humidity it picks up a fraction of a percent of moisture.  That is all tied to the purity of the potassium nitrate used.  When GOEX got away from the Vicksburg Chemical fertilizer grade potassium nitrate in 2000 it made a vast improvement in their powder as far as moisture pick up goes.  This was the main problem wityh 4f priming powder made prior to the switch to a better grade of potassium nitrate that comes in from Chile.
A 99.9% pure potassium nitrate picks up only fractions of a percent of moisture when the R.H. is below 92%.  The 92% R.H. is something of turning point in the hygroscopic behavior of potassium nitrate.  At 99% R.H. you would be looking at a maximum moisture content, in the powder, around 1.5%.  If one subjected the Vicksburg Chemical Company potassium nitrate to the same conditions you would be looking at 16% moisture content at 99% R.H.

With the brands of black powder we have today your biggest problem in a hunting load is that the first shot out of a clean barrel will gives velocities lower than if you had fired the shot out of a "dirty" barrel.  If you normally target shoot 80 grains of 3F in a .50 caliber flinter you might need 90 grains to get the first shot velocity up to that you see with repeated shots even though you wipe the bore between shots.

Bill K.

[The other DWS]

HUH?    a "dirty" barrel shooting faster than a clean one.   that sounds real counterintuitive to me.

unless

I could see that maybe a warmed barrel might make a diff.  but I suspect that might be a function of barrel temp and lubed patch friction.  Warm, still moist, fouling might be slicker than a cold clean barrel.  Is that what you are referring to?

I shoot a lot of ASSRA single shot rifle matches with 22rf.  And at 200 yards on the cool/cold days of our Spring 22 rf-only shoot it can take up to 10 rounds to get the barrel warm enough for the bullet lube coating in the barrel to be slick for the full length of the bore.

[Mad Monk]

HUH?    a "dirty" barrel shooting faster than a clean one.   that sounds real counterintuitive to me.

unless

I could see that maybe a warmed barrel might make a diff.  but I suspect that might be a function of barrel temp and lubed patch friction.  Warm, still moist, fouling might be slicker than a cold clean barrel.  Is that what you are referring to?

I shoot a lot of ASSRA single shot rifle matches with 22rf.  And at 200 yards on the cool/cold days of our Spring 22 rf-only shoot it can take up to 10 rounds to get the barrel warm enough for the bullet lube coating in the barrel to be slick for the full length of the bore.

A patched ball is easily accelerated in the bore as the powder begins to burn and evolve gases.  Anything in the bore that adds resistance to the initial movement of the ball delays ever so slightly its acceleration.  You then get more pressure behind the ball which gives a faster rate of acceleration part way up the bore.

I used to look at bore fouling properties of test powder buy shooting from a clean barrel and watch the pattern of increasing velocity to a point where the increase levels off.  Then watch the shot to shot variation.

When I first shot the Swiss powder in my .50 caliber rifle I wiped between shots.  A big ES resulted.  Then I shot it again without wiping the bore.  Single digit ES.  The powder found its own "steady state" of fouling in the bore.  All part of the moist-burning property of that powder.

Even traces of fouling in the bore can give velocity variations with differences in the amount of trace fouling.

I used to deal with a noted bench gun shooter.  Between shots he cleaned the bore squaky clean to get uniform shot to shot velocities that gave him winning groups.

Bill K.