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Thread: Gas port erosion with different barrel profiles

  1. #21
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    Quote Originally Posted by ether View Post
    I sincerely apologize to the board and to MistWolf for my behavior...
    Apology accepted and appreciated. Think nothing more of it

    Quote Originally Posted by michael word View Post
    ...If you, however, do alot of mag dumps the thinner barrel will most likely wear in all areas faster than a thicker barrel because of the increased heat...
    With the same round count in the same amount of time, the heat is actually the same. What differs is the thinner barrel (in this case means lesser mass) has less material with which to absorb the heat and less surface for dissipation. With the same amount of heat (BTUs) the barrel with less mass will reach a higher temperature.

    Think of the barrel of a rifle like, well a barrel. Take a 50 gallon barrel and cut a 2 inch diameter hole in the bottom, then take say a 25 gallon barrel with a 1 inch hole in the bottom. (We put a larger drain in the 50 gallon barrel because the larger diameter rifle barrel has more surface with which it can "drain" the heat.) Now fill both with water at a rate of 5 gallons per second. The 25 gallon barrel will fill up faster, and you will have to stop more often to wait for it to drain before adding any more water without it overflowing. The 55 gallon barrel will take longer to fill and drains at a slightly faster rate but will have more water to drain before it's completely empty. It's the same with rifle barrels and heat.

    What erodes the gas port is gas velocity, density and temperature. High velocity gas cuts very well and when the steel gets hot, it is less resistant to gas cutting. (Gas leaking pas a bullet is also gas cutting and contributes to the wear of the bore.)

    The reason there is no gas cutting on the opposite side of the port is due to the reduction of the gas velocity. The gases enter the port at supersonic speeds but in passing through the port, which is a venturi, the velocity is reduced to sub-sonic speeds. (Supersonic speed fluids passing through a venturi behave differently than subsonic speed fluids.)

    So, a thinner barrel at the gas port may be more susceptible to port erosion if the thinner material leads to higher temperatures at that location. Also, a barrel thicker at the port will have more material to cut through before enlarging the diameter on the opposite side.

    It's also possible that distance of the port from the chamber would have even a greater affect. The closer to the chamber, the higher the temperature and the greater the density of the gases hitting the port
    Last edited by MistWolf; 01-15-13 at 10:01.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  2. #22
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    Quote Originally Posted by MistWolf View Post
    With the same round count in the same amount of time, the heat is actually the same. What differs is the thinner barrel (in this case means lesser mass) has less material with which to absorb the heat and less surface for dissipation. With the same amount of heat (BTUs) the barrel with less mass will reach a higher temperature.

    Think of the barrel of a rifle like, well a barrel. Take a 50 gallon barrel and cut a 2 inch diameter hole in the bottom, then take say a 25 gallon barrel with a 1 inch hole in the bottom. (We put a larger drain in the 50 gallon barrel because the larger diameter rifle barrel has more surface with which it can "drain" the heat.) Now fill both with water at a rate of 5 gallons per second. The 25 gallon barrel will fill up faster, and you will have to stop more often to wait for it to drain before adding any more water without it overflowing. The 55 gallon barrel will take longer to fill and drains at a slightly faster rate but will have more water to drain before it's completely empty. It's the same with rifle barrels and heat.

    What erodes the gas port is gas velocity, density and temperature. High velocity gas cuts very well and when the steel gets hot, it is less resistant to gas cutting. (Gas leaking pas a bullet is also gas cutting and contributes to the wear of the bore.)

    The reason there is no gas cutting on the opposite side of the port is due to the reduction of the gas velocity. The gases enter the port at supersonic speeds but in passing through the port, which is a venturi, the velocity is reduced to sub-sonic speeds. (Supersonic speed fluids passing through a venturi behave differently than subsonic speed fluids.)

    So, a thinner barrel at the gas port may be more susceptible to port erosion if the thinner material leads to higher temperatures at that location. Also, a barrel thicker at the port will have more material to cut through before enlarging the diameter on the opposite side.

    It's also possible that distance of the port from the chamber would have even a greater affect. The closer to the chamber, the higher the temperature and the greater the density of the gases hitting the port
    Thanks for this explanation, MistWolf.
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    We miss you, ToddG.

  3. #23
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    Quote Originally Posted by MistWolf View Post
    Apology accepted and appreciated. Think nothing more of it



    With the same round count in the same amount of time, the heat is actually the same. What differs is the thinner barrel (in this case means lesser mass) has less material with which to absorb the heat and less surface for dissipation. With the same amount of heat (BTUs) the barrel with less mass will reach a higher temperature.

    Think of the barrel of a rifle like, well a barrel. Take a 50 gallon barrel and cut a 2 inch diameter hole in the bottom, then take say a 25 gallon barrel with a 1 inch hole in the bottom. (We put a larger drain in the 50 gallon barrel because the larger diameter rifle barrel has more surface with which it can "drain" the heat.) Now fill both with water at a rate of 5 gallons per second. The 25 gallon barrel will fill up faster, and you will have to stop more often to wait for it to drain before adding any more water without it overflowing. The 55 gallon barrel will take longer to fill and drains at a slightly faster rate but will have more water to drain before it's completely empty. It's the same with rifle barrels and heat.

    What erodes the gas port is gas velocity, density and temperature. High velocity gas cuts very well and when the steel gets hot, it is less resistant to gas cutting. (Gas leaking pas a bullet is also gas cutting and contributes to the wear of the bore.)

    The reason there is no gas cutting on the opposite side of the port is due to the reduction of the gas velocity. The gases enter the port at supersonic speeds but in passing through the port, which is a venturi, the velocity is reduced to sub-sonic speeds. (Supersonic speed fluids passing through a venturi behave differently than subsonic speed fluids.)

    So, a thinner barrel at the gas port may be more susceptible to port erosion if the thinner material leads to higher temperatures at that location. Also, a barrel thicker at the port will have more material to cut through before enlarging the diameter on the opposite side.

    It's also possible that distance of the port from the chamber would have even a greater affect. The closer to the chamber, the higher the temperature and the greater the density of the gases hitting the port
    Exactly what i was getting at, just in less words. Good explination. However, I have to disagee of the heat disipation. The larger barrel will take longer to cool due to the increased mass than the thinner barrel. I experience this at work every day as a machinist working on everything from 2-8" diameter pipe. The larger the pipe, the longer I have to wait before I can continue to the next step.
    Last edited by michael word; 01-15-13 at 15:45.

  4. #24
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    You are correct that the temperature of the larger barrel will take longer to cool to room temperature than the smaller. The reason is because it contains more heat (more BTUs). Because the larger diameter barrel has more surface, it will radiate the heat faster than the smaller one, but because it takes more BTUs (more heat) to reach the same temperature, it will take more time to shed that heat, just as it took more time to reach that temperature.

    It's also about surface to mass ratio. Take a piece of aluminum foil, lay it flat in the oven and heat it to 450 degrees and it will heat up right away and cool off right away. Ball it up and do the same thing, it will take more time to heat up and cool off although it has absorbed the same amount of BTUs to bring it's mass to the same temperature

    You also know that the smaller pipe when you machine it takes less heat before it gets hot. That's because the smaller pipe has less mass to absorb the BTUs. That's why you can heat a paperclip with a cigarette lighter until it turns cherry red. But if you had a 1 lbs cube of the same material and took the same lighter to it, the best to hope for would be to make it warm to the touch.

    I'm not trying to tell you that you don't know your stuff, because it's obvious that you do. Just trying to shed light on how heat (BTUs) and mass affect temperature
    Last edited by MistWolf; 01-15-13 at 16:27.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  5. #25
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    Quote Originally Posted by MistWolf View Post
    You are correct that the temperature of the larger barrel will take longer to cool to room temperature than the smaller. The reason is because it contains more heat (more BTUs). Because the larger diameter barrel has more surface, it will radiate the heat faster than the smaller one, but because it takes more BTUs (more heat) to reach the same temperature, it will take more time to shed that heat, just as it took more time to reach that temperature.
    So I'm thinking that a pencil barrel could arguably be the best "all-around" barrel for semi-auto-only applications, as long as you aren't torturing it like in the 40K video. In an everyday utilitarian situation, your firing is likely more sporadic than sustained. Port erosion should be negligible. The barrel is getting hotter faster, but it's not insulating that heat as much in the bore, and it's cooling off faster on both the inside and outside of the barrel...ideal in case you need to shoulder the weapon and move around...not to mention the weight savings which go without saying...
    Last edited by ether; 01-15-13 at 20:19.

  6. #26
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    correct.

  7. #27
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    Bottom line-

    Fire 30 shots in thirty seconds with a pencil barrel
    Fire 30 shots in thirty seconds with a heavy profile barrel.
    Both barrels will absorb the same amount of heat. Take the temperature of both. The pencil barrel will have a higher temperature. The heavy barrel will cool to ambient temperature a little faster because it has greater surface to radiate the heat.

    Now-

    Fire one round per second in both barrels until both barrels reach 200 degrees. The pencil barrel will reach 200 degrees with fewer shots fired than the heavy barrel. The pencil barrel will also cool faster than the heavy barrel because it is holding less heat.

    Both barrels will radiate heat inside the bore. That's why knowledgeable shooters will put their rifles aside with the bolt open and the muzzle up (or down) to let air flow through the barrel to aid in cooling
    Last edited by MistWolf; 01-16-13 at 08:40.
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  8. #28
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    Cool, I have my answer. I really am sorry for the way I started this thread, but in spite of me it turned out to be one of the more informative I can recall...many thanks all.
    Last edited by ether; 01-15-13 at 23:17.

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