I know that you can “sweat” a bearing race onto a shaft by heating the bearing with a torch. Both ID and OD will expand, and the inner race will drop onto the shaft, then tighten when the bearing cools. You can also remove them the same way with heat.
The ID of the barrel increases. Think about shrink fit. You heat a piece of metal so that the hole gets bigger in order to fit over the cooler stud. Then, as the metal cools, the ID gets smaller and tighter around the stud.
Also, think about it this way. When metal (or any material for that matter) gets warmer, it expands. So, think about that in terms of the hole. You would have expanding metal trying to cram into a smaller space (the hole). Doesnt work like that.
Technically, from a physics point of view. the only way the ID of a hole could get smaller is if there was a major temperature difference between different areas of the metal, in that the heat is localized and thus the expansion has nowhere to go but into the ID. Unfortunately, barrels are not thick enough to allow that to happen and there is nothing impeding the transfer of heat through the barrel.
As for the muzzle flash, I think it has more to do with the temperature of the cartridge prior to firing. Think about how pressure increases as temperature increases (yes, I know powders are a lot more temperature insensitive today) than they used to be, but as the heat of the barrel increase the heat of the cartridge. If you dont think that short time in the chamber can impact it that quick, go to youtube and check out the torture test where they attempt to melt down their surge with an automatic M4. Even at full auto, before the suppressor fails, they are cooking rounds off before the rifle goes into full battery. So, I have a feeling that is the cause of the increased muzzle flash.
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Well gents, I’d say we’ve settled this topic. Expanding barrel and excess heat make a bigger flash. I learned something from here so it’s been productive!
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