Temp and humidity effects on internal ballistics

[dgeesaman]

I’ve read a lot of commentary that temps and humidity affects load performance. But I want to better understand how it changes internal ballistics:

1) When we talk about humidity, I assume we are only talking about humidity of the powder and air when the bullet is seated? Seems like loading in a controlled environment could / should eliminate this variable. Humidity is sealed once the bullet is seated.

2) When we talk about temps, I assume we’re talking about the temperature of the powder in the cartridge at the moment it’s ignited? If so, then keeping the cartridge warm (say 80f) and firing it in a warm chamber (say, Luke warm to the touch) should produce the same velocity as an 80f day. Or, shooting with cold cartridges in a warm chamber, means the amount of time with the round chambered will affect its combustion too.

3) External ballistics obviously follow the ambient conditions. I have no questions about it.

 

[savagedasher]

Most of the people load in a air conditioned room and store the powder there
So in most cases that problem is solved
When tuning the water grain of air
Effect the way a bullet react in flight
Her in Florida our bullet slow fast then it doesn’t in say Ohio
Any deviation of angle effects the bullet drop
If you shoot down hill 20 degrees
The bullet drop will be less
Or It will be like shooting shorter yartage
Air density has the same effect

 

[Turbulent Turtle]

Most of the people load in a air conditioned room and store the powder there
So in most cases that problem is solved
When tuning the water grain of air
Effect the way a bullet react in flight
Her in Florida our bullet slow fast then it doesn’t in say Ohio
Any deviation of angle effects the bullet drop
If you shoot down hill 20 degrees
The bullet drop will be less
Or It will be like shooting shorter yartage
Air density has the same effect

I'm not sure I understand what you are saying in the bolded text above, but I think you're trying to say that bullets in Florida slow down faster than in Ohio.

That would only happen if for some strange atmospheric reason it would be hotter and more humid in Ohio than in Florida, at equivalent elevation.

Here in south Texas, we know our bullets fly faster and higher when it's 99F and 100% humidity as compared to the depth of winter 32F and 20% humidity, when we have to take a running start before pressing the trigger to get some velocity.

 

[dgeesaman]

All true: warmer, higher altitude, and more humid air should let bullets fly longer and faster. It’s certainly true with golf balls. But these are still external ballistics. I think we’re ready to go back to my questions about internal ballistics.

 

[Turbulent Turtle]

I’ve read a lot of commentary that temps and humidity affects load performance. But I want to better understand how it changes internal ballistics:

1) When we talk about humidity, I assume we are only talking about humidity of the powder and air when the bullet is seated? Seems like loading in a controlled environment could / should eliminate this variable. Humidity is sealed once the bullet is seated.

2) When we talk about temps, I assume we’re talking about the temperature of the powder in the cartridge at the moment it’s ignited? If so, then keeping the cartridge warm (say 80f) and firing it in a warm chamber (say, Luke warm to the touch) should produce the same velocity as an 80f day. Or, shooting with cold cartridges in a warm chamber, means the amount of time with the round chambered will affect its combustion too.

3) External ballistics obviously follow the ambient conditions. I have no questions about it.

1-irrelevant unless you're loading underwater.

2- correct. Which is why we like temp-insensitive powders such as Varget.

3- ok.

 

[savagedasher]

I'm not sure I understand what you are saying in the bolded text above, but I think you're trying to say that bullets in Florida slow down faster than in Ohio.

That would only happen if for some strange atmospheric reason it would be hotter and more humid in Ohio than in Florida, at equivalent elevation.

Here in south Texas, we know our bullets fly faster and higher when it's 99F and 100% humidity as compared to the depth of winter 32F and 20% humidity, when we have to take a running start before pressing the trigger to get some velocity.

I can tell.clay bird have to start out faster in Florida then Ohio
ATA did a test From Miami to Dayton Ohio
The same machine The clay bird went 35 yard farther in Ohio

 

[Turbulent Turtle]

I can tell.clay bird have to start out faster in Florida then Ohio
ATA did a test From Miami to Dayton Ohio
The same machine The clay bird went 35 yard farther in Ohio

It's the same reason why planes take off faster when it's cold.

 

[chkunz]

For internal ballistics, no significant effect.

 

[dgeesaman]

2- correct. Which is why we like temp-insensitive powders such as Varget.

So then I can continue messing around in winter developing loads as long as I keep the cartridges and chamber warm.

Not match quality loads, but certainly hunting quality loads.

David

 

[dcali]

Some of the benchrest shooters, correct me if I’m mistaken, but isn’t the humidity concern more about throwing consistent charges at the range than any impact on powder combustion or external ballistics?

 

[Ned Ludd]

So then I can continue messing around in winter developing loads as long as I keep the cartridges and chamber warm.

Not match quality loads, but certainly hunting quality loads.

David

If you think you can realistically reproduce the heat in both the barreled action, as well as the ammunition, it might work. Otherwise, no. A significant portion of the powder burn occurs outside the case in the first 10-12 inches of barrel. When that barrel gets hots, it will noticeably affect burn rate and velocity. Due to mass action, simply heating (or cooling) loaded rounds prior to inserting them in the chamber will not dramatically change the temperature of enormous [relative] mass of the barreled action. Further, brass is a very good conductor of heat. You simply can't close the bolt and pull the trigger fast enough to prevent a change in temp of the loaded round if the barreled action is at a different temperature than the loaded round.

If you don't have access to a shooting building or tent where you can control the ambient temperature to some degree, you're better off working up loads at both the high and low end of the temperature range you'll most commonly shoot in at different times of the year.

 

[dgeesaman]

Some of the benchrest shooters, correct me if I’m mistaken, but isn’t the humidity concern more about throwing consistent charges at the range than any impact on powder combustion or external ballistics?

That would much better explain the comments I’ve heard about humidity affecting a given powder.

 

[dgeesaman]

If you think you can realistically reproduce the heat in both the barreled action, as well as the ammunition, it might work. Otherwise, no. A significant portion of the powder burn occurs outside the case in the first 10-12 inches of barrel. When that barrel gets hots, it will noticeably affect burn rate and velocity. Due to mass action, simply heating (or cooling) loaded rounds prior to inserting them in the chamber will not dramatically change the temperature of enormous [relative] mass of the barreled action. Further, brass is a very good conductor of heat. You simply can't close the bolt and pull the trigger fast enough to prevent a change in temp of the loaded round if the barreled action is at a different temperature than the loaded round.

I think I agree with all of this.

My observations (no measurements) suggest that keeping the ammo warm plus adding it to a warm chamber will keep it sufficiently warmer than ambient.

One advantage to shooting in warm weather is with enough time you can always wait long enough and everything will creep back down to ambient temperature.

In winter, after my fouling shots, the chamber can be kept warm to the touch, and firing rate can be adjusted to let each round spend some time in the chamber. In shifting winds though it gets more tricky because wind conditions are more critical and sometimes I have to fire early or wait a while.

 

[dcali]

If you really want to shoot warm round during the winter and have access to power, here's a thought:

Put a reptile terrarium heater (you can get them at PetSmart or similar) in a cooler, and using an Inkbird temperature controller (you can get them on Amazon for about $35) to keep your ammo warm. You can control it down to a couple degrees, and the total expense is less than $100. I picked this trick up making beer. Works like a charm (for beer - I've never tried for bullets). You might need to add a thermal mass (a big rock or something like that) to the cooler to keep it stable. You don't need huge wattage - the outlets in a truck will do. Just make sure the ammo gets stable at the temp you want before you leave for the range - it doesn't happen quickly and you want to make sure it's soaked all the way through.

I'm not sure I agree that the ammo would heat up in the gun - although I have no data to support this speculation. Brass heats up fairly quickly in a hot barrel, but powder has a very low amount of surface contact with the inside of the brass, so it will heat primarily through radiation. I would *guess* that powder tempeature lags brass temperature significantly (on the time scale of loading and firing, at least).

 

[Turbulent Turtle]

So then I can continue messing around in winter developing loads as long as I keep the cartridges and chamber warm.

Not match quality loads, but certainly hunting quality loads.

David

If by developing loads you mean assembling them at home and shooting them in winter, then sure but I want to throw two big caveats:

1- You need to keep the ammo at a reasonable temperature. In summer time, we do not leave out ammo directly exposed to the sun. I have seen and heard about people who left their ammo boxes in the sun and came back from the pits to shoot their maximum loads, now very warm and experience very sticky extraction and surprises on the target. I would not let my ammo cold soak and then shoot it and use that to develop near-max loads to be used in summer. I use Varget, a temp-insensitive powder, and I never leave it in the sun. If I were to shoot in very cold conditions (<20F) I would keep it an insulated bag or box.

2- The speed of sound is only affected by temperature, nothing else. (Well maybe humidity a slight bit, but inconsequential for us.) I think it's 1.5 FPS per degree F, but you can look it up. On the other hand, cold dry air is denser than warm humid air, this means that bullets which are barely supersonic at the target in warm weather, will go subsonic in cold weather. (Ask me how I know this.) Plus if you let you ammo get cold, that only gets worse.

So, developing a max load on a cold winter's day using cold ammo and then shooting that ammo on a hot summer day after leaving the ammo to bake in the sun is not the wisest course of action.

 

[dgeesaman]

Maybe this is old hat to some of you but I’m glad you’re all chipping in and helping.

So far no surprises from me as this follows basic heat transfer behavior and the external ballistic concerns (speed of sound, air density,etc) are entirely logical and even calculable.

I agree it would be foolish to attempt to develop a max load in cold conditions. I think developing mild to medium loads should be very reasonable if ammo temp is regulated and loading is done in a relatively dry environment.

Maintaining a consistent ammo temp is pretty straightforward, in my little world. I suspect most other shooting environments are more challenging.
- I load indoors and store my range bag in a heated area.
- Its a 5 or ten minute drive to either range I use. Ammo box stays in the range bag and doesn’t feel the cold car, at least until the car is warmed up.
- I can park close to the firing line. On some days I leave the ammo box and bring a group at a time to the line. I can let my car run and heater running.
- I literally keep in touch with barrel and chamber temps. I can regulate firing rate so that chamber and barrel stay between room temp to moderately warm. I agree with Damoncali, the powder and air temp inside the cartridge should respond slowly to the brass warming or cooling, which helps. I suspect it would take minutes in a warm chamber to affect the powder burn rate.
- I have a parka with a generous inside chest pocket. If the car isn’t convenient, I keep the ammo box there.
- I’m shooting 200y range max at 3000ish FPS muzzle velocity. Transonic effects and major BC changes shouldn’t come into play.
- I always keep my current group of rounds in a pocket that stays warm. I don’t shoot round-robin in this situation so they don’t get mixed up.
- in summer i keep ammo out of the sun. These ranges are covered so that’s easy.
- Choosing less temperature sensitive powders is obviously superior for these kinds of loads and conditions.

Thanks, David