Originally Posted by
MistWolf
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