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Thread: hot barrels, hot dogs and hot babes

  1. #1

    Question hot barrels, hot dogs and hot babes

    We all know that chamber pressure rises when we shoot in real hot weather, but my question is why? Is it because of a hot barrel, cartridge,or both? Does the powder being hot play a role? When the barrel expands, does it expand evenly and does the bore get smaller? If the bore gets smaller and bullet expands, the bore/bullet relationship becomes tighter, and is that where the pressure comes into play? I have my own theories, but I don't know if I've ever seen it in print. Oh...the hot dogs and babes? Just seeing if you're awake. Thanks, ciao.

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    New member George's Avatar
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    Default hot shooting

    Awake? barely. But I think that barrels warm and cool unevenly- how unevenly? don't know. They do expand with heat and that does decrease bore size. How warm the ammo is depends on how warm it is before going into the chamber plus how long it sits in a warm chamber before being fired. All changing variables that affect shooting and gun dymanics... pressure, group size, velocity, accuracy, POI and the like. Yes, hot barrels mean smaller bores and hot bullets mean larger bullets... so more pressure. The other part is primer/ powder ignition and burn rates. I've always thought of that part of the shooting process as a chemical reaction where heat acts as a catalyst to the reaction. More heat equals faster ignition and faster burn rate so also more pressure. My guess anyway

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    Fact...When a bore or cylinder heats up it expands. The bore becomes LARGER! INDISPUTABLE!!!

    Concept is understood in many industries from racing through precision machinery. The only way the bore can get smaller is for the metal to shrink as when the temperature drops. For the bore to get smaller the inner surface circumference has to shrink, for the bore to get larger the inner surface cicumference has to expand.

    Powder is temperature sensitive. The higher the temperature the faster the burn rate.

    Higher temperture will expand the bullet.

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    Moderator Paul H's Avatar
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    My understanding is the powder burns faster in higher temperatures, and thus the pressure peak occurs earlier and is higher than in colder temperatures.

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    Quote Originally Posted by Paul H View Post
    My understanding is the powder burns faster in higher temperatures, and thus the pressure peak occurs earlier and is higher than in colder temperatures.
    Yep! that is it!
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  6. #6

    Talking when yer hot

    Quote Originally Posted by Ed M View Post
    Fact...When a bore or cylinder heats up it expands. The bore becomes LARGER! INDISPUTABLE!!!

    Concept is understood in many industries from racing through precision machinery. The only way the bore can get smaller is for the metal to shrink as when the temperature drops. For the bore to get smaller the inner surface circumference has to shrink, for the bore to get larger the inner surface cicumference has to expand.

    Powder is temperature sensitive. The higher the temperature the faster the burn rate.

    Higher temperture will expand the bullet.
    Hi Ed: Your statement about the bore expanding is what another fellow said, that brought on this thread. This guy said if that was the case then there should be less pressure because the bullet/bore relationship would not be as tight. That being true, then it must be in the cartridge to cause it. Then by theory, if one keeps the cartridges on ice before firing, then could a case be made that there would then be lower than "normal" pressures...hence, larger bore and smaller dia. bullet?

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    Member Float Pilot's Avatar
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    I have loaded hot handloads during the winter and fired them at 10 to 20 below zero. They were hot but showed no signs of excess pressure.
    I took some of the same loads down to Nevada and fired them on a hot (for me hot is anything above 65) day. One primer blew out and I also jammed the next shot in the chamber after firing. It must have been in the 80s since I was about ready to pass out. Those loads when into the trash.
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  8. #8

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    Quote Originally Posted by Float Pilot View Post
    I have loaded hot handloads during the winter and fired them at 10 to 20 below zero. They were hot but showed no signs of excess pressure.
    I took some of the same loads down to Nevada and fired them on a hot (for me hot is anything above 65) day. One primer blew out and I also jammed the next shot in the chamber after firing. It must have been in the 80s since I was about ready to pass out. Those loads when into the trash.
    Yes!!!!! you must adjust loads to conditions that are extreme. Extreme cold less pressure less velocity, Extreme hot (Africa, Western US deserts) more pressure more velocity. Adjust loads unless you are hunting above 20 and under 80 degrees and this depends also on certain powders and also the rifle will dictate what happens. Now if you load 5% under max don't worry just go hunt. Now this is from years of shooting and hunting always checking my brass, primers, group size, and even measuring the base of my brass to note the expansion. I have certain powders that shoot well when the temp is nearing the extreme and I trust these powders, because they have always preformed for me with no suprises if I watch how and what I load for a given hunt or shoot and know the general temp for the area and time that I am shooing or hunting. Do not us reduced loads beyond what a reloading book says, if you do you might be surpised at what can happen and some sure have been suprised. Just use moderate to 5% below max if you are not sure when you handload and the temp is real hot or real cold.
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    New member George's Avatar
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    Default yes on cylinder dimension

    Ed M... thanks, that correction is exactly right I think. It's the differential between the coefficients of thermal transverse expansion between the bullet and the barrel that matters if looking at bullet friction/load pressure.

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    The equation for the calculation of rate of expansion uses atmospheric pressure and temperature. This is the temperature of the powder in the cartridge, and the ambient air around it, at the time of ignition. The heat from a hot barrel will raise the temperature of the loaded cartridge or the heat from a round laying on the dash of the truck on a hot day. However the cartridge gets hot, doesn't matter. The rate of expansion of the gases produced by the powder will increase when temp is higher and peak pressure will be reached quicker and will be higher than when fired at a cooler temperature.

    Some powders are more responsive to temperature changes than others. The various double base ball powders are considered the worst, or are more temperature sensitive. The IMR series of powders in the old days anyway, were much better. Now the best powder or the least temperature sensitive are the Hodgdon's Extreme series of powders. The Reloader powders were always very good but have been dethroned by the new Hodgdon powders. Vihta Vouri powders are also very temperature stable.

    I have been doing a lot of winter summer testing in Fairbanks and with a 100 degree F temperature range or more (-40 to +70F). Some powders in magnum rifle give velocity (and obviously pressure) variation as much as 150 fps and others give no more than 40 fps. Some powders will cause hangfires at -40 even in small capacity cases. At -20 ish mostly powders do well, it's that last 20 degrees that seems to matter most. I have some data with about 6 or 8 powders and about a half dozen calibers within these temperatures. Now I have to change primers and do it all again.
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    So will keeping the cartridge cool keep the pressures down even on a hot day? It seems logical, but...

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    Quote Originally Posted by Ripper View Post
    So will keeping the cartridge cool keep the pressures down even on a hot day? It seems logical, but...
    Yes, but if the air is still hot there is a difference, but not really noticable. Heating the ammo in the sun will definately show it's ugly head if the load is max at normal temps.
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    The same thing applies to solid rocket boosters. Wild variations in temps can cause them to produce unexpected amounts of thrust.
    Missle storage magazines have their temp recorded once or twice a day.
    Those temps are forwarded to some poor slob who has to figure the usefull life of the boosters based upon years of temp variations.
    Ship-board magazines have cooling and heating systems to help keep the weapons within a reasonable comfort range.
    The old MK 16 ASROC launchers also had heating / cooling lines embedded in the sides of the missle cells. Each cell had a thermometer used to keep track of the ASROC or Harpoon missles that were loaded into those cells.

    Trying to keep them cool was a real pain since the heat exchanger used sea water to cool the antifreeze running through the system. In a place like the Persian Gulf the sea water might be 90 degrees. So not much heat was exchanged when the air temp was 115.
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  14. #14

    Talking high pressure/cold temp

    Quote Originally Posted by Murphy View Post
    Yes, but if the air is still hot there is a difference, but not really noticable. Heating the ammo in the sun will definately show it's ugly head if the load is max at normal temps.
    Hi Murphy: I noticed that the speer manual stated that pressure increases again when temps go below -20 with double based powders. True? I shot a muskox up in the Artic one year, and it was -30 to -40 everyday, but I kept the shells in my pocket until it was time to shoot...don't know if it helped, but it sure didn't hurt. What I read was the reason.

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    Quote Originally Posted by Maydog View Post
    We all know that chamber pressure rises when we shoot in real hot weather, but my question is why?
    Propellant is a chemical. Heating the chemical compound increases the reaction rate, as does pressure. This is why propellant burns slowly and inefficiently in air, but rapidly and efficiently when confined.

    What is happening is the molecules are getting closer to their activation energy (the temp at which point the reaction commences), so as the average energy is increased, the molecules require less energy to be absorbed to become involved in the reaction, so more molecules are available more quickly.

    That's the simple explanation.

  16. #16

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    Quote Originally Posted by Nitroman View Post
    Propellant is a chemical. Heating the chemical compound increases the reaction rate, as does pressure. This is why propellant burns slowly and inefficiently in air, but rapidly and efficiently when confined.

    What is happening is the molecules are getting closer to their activation energy (the temp at which point the reaction commences), so as the average energy is increased, the molecules require less energy to be absorbed to become involved in the reaction, so more molecules are available more quickly.

    That's the simple explanation.
    I would say that is a very concise means of wrapping it all up. Like it!
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    In addition to what nitroman said;

    Basically, nitrocellouse can produce its own oxygen on ignition for combustion..hence, yes you can fire a bullet in space theoretically.

    The hotter air is, the less dense it is, and the faster this chemical reaction (which is basically expansion on a rapid scale) can happen. So faster expansion in the same area of confinement (bore volume) is higher pressure from higher speed of created volume (gasses).

    This is different (opposite)from internal combustion engines where oxygen is needed to be introduced into the equation and colder air is more dense thereby containing more oxygen and producing more power.

    hope this helps.

    jedi

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