Bullet Temperature

  • This may be a really stupid question, but I was wondering why my 5.56 55 grain bullets are hot enough to burn your skin after being fired and my .308 175 grain shells are barely even warm?

  • We talking about the cases? Not sure how you're grabbing bullets that quickly.

    If talking cases, are we talking an AR15 for the 5.56 and a bolt gun for the 308?

  • Yes, AR-15 5.56 and Bolt .308 cases. Sometimes I'll just hold my hand over bolt and catch the case when it ejects and it's barely warm. On the AR-15 a friend of my sons was kinda burnt when a case bounced of the indoor range wall and land in his shirt and I have picked the cases off the ground after firing the AR-15 and they are Hot.

  • Is the 556 rifle an AR? All that gas comes back into the chamber via the gas tube. Hot.

    Also the easy answer to me is the 223 brass has less mass and runs at higher pressure than the 308 brass.

    For a equal volumes of gas, temperatures will increase directly proportionate to the pressure increases. Obviously the about 25 grains of powder in a 223 case will generate less gas than the 308 with its 45ish grains but it will be hotter based on its operating pressure. This relationship is expressed in the combined gas law P1V1T2=P2V2T1. Assuming pressure is held constant, Simply cross out the V's and you see the relationship between temp and pressure

    Also heat capacity. A random 223 piece weighs 6.3 grams and a random 308 weighs 11.4. Because I'm super lazy, I'm just gonna say the 308 is double the mass of the 223. Brass has a heat capacity of .38 Joules per gram degree Celsius, or in order to raise one gram of brass one degree it takes .38 joules.
    Thus in order to raise the temperature of the two different pieces of brass to the same temperature it's gonna take double the energy for the 308.

    If we get more into it we could calculate it more precisely but we'd need the temperature and volume and mass of the two different pieces of brass.


  • Thank you, that makes total sense. Was just an "I wonder why question", that we were wondering about. No further calculations required.

  • @ragnarnar said:

    Also the easy answer to me is the 223 brass has less mass and runs at higher pressure than the 308 brass.

    Incorrect sir. The pressures at play in .223 and 308 are virtually identical.


    The reason the 223 is hot is because it's a semi auto. The bolt unlocks early to facilitate ejection and uses the pressure of the gas to eject the round. This creates a TON of heat dump into the chamber and into the bolt. Even on piston systems, the hot gas finding its way past the cartridge from the barrel as the chamber is unlocked early is still enough to create a lot of heat.

    A semi auto 308 would create the same effect as the semi auto 5.56. A .223 bolt gun produces brass the same relative temperature as a 308 bolt gun.

    @ragnarnar Remember this is not sniperforums.com. Your answer was very technical and would lead some people to believe you, despite being 100% the wrong answer. The first part of your answer was correct. The second part was total hogwash. Don't take this personally. Just remember that here we need to actually know, not just "seem" like we know. ;)

  • Thank you! Another on of life's "I wonder why questions answered".

  • @orkan

    Not personally at all. I legitimately thought the pressure was higher in 223. My bad folks.

    I probably heard that on sniper forums🙄

    The AR factor explains the massive difference in temperature. The complex gas chemistry only explains it if your rounds are at different pressures.

  • I would like to throw something in here also...
    It is my understanding that a loose chamber will also eject hotter cases than a tight chamber as the barrel sucks off a lot of the heat from the case.

  • Is that graph from QuickLoad?

    An adjustable gas block and an IR thermometer would be interesting to experiment with.

  • @hypo said:

    Is that graph from QuickLoad?