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I've wondered about this before but Art's thread on resizing freshly annealed brass brought it back up front in my head.

Let's take two .308 cases. One is fired in an M1A and the other in an M700 Remington.

We all know the brass coming out of the M1A is too hot to pick up for a few seconds after ejection - so the brass cools down slowly over some seconds.

The brass coming out of the M700 Remington has cooled down greatly in comparizon and can be held in your hand.

Does the time at higher temperatures have a different effect on the brass from the M1A vs. the M700?
 

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I don't believe the brass is getting hot enough to anneal it. I do know the brass is still expanding when it gets ejected from an M14/M1A. That is the main reason it is harder on brass than bolt guns.
 

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The physics here are a little different. If the two cartidges have the same powder, powder charge, primer, and weight bullets and being shot with similar length barrels the temperatures inside the cases will be almost the same. The difference is in chamber size and action of the rifles is what causes temperature variance of extracted cases.
Bolt actions have tighter chambers. When the cartidge is fired, the case expands to the chamber size and transfers more heat to the rifle. Also when the expanding gas leaves the barrel, ambiant air rushes back in causing the case to contract so it can be extracted. This all happens in miliseconds. Plus the time to pull the bolt back helps cool it.
On the M14/M1A the chambers are larger. So as the cartridge is fired, the case is starting to expand it is not making as much contact to the chamber walls(Less heat transfer) when it gets yanked out of the chamber. This causes more heat to be retained in the case. If you watch the case extraction videos, you will see smoke coming out of both the chamber and case.
It is the actions of the rifles that make the difference.

Glenn
 

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I thought that brass started to contract toward original size before extraction/ejection started. The brass expands to completely seal the chamber, preventing gas blowback, and potential injury to the shooter. The hot gasses go through the port, and the timing on this action allows the brass to start contraction; allowing easy ejection. Correct timing prevents a torn cartridge rim, or head separation.

Early M16's had this problem when authorities sought to increase the cyclical rate of the M16, using hotter powder.

The M1A/M14 chamber is a bit roomier, but not to the point that the brass doesn't expand to fill it.
 

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I thought that brass started to contract toward original size before extraction/ejection started. The brass expands to completely seal the chamber
I am no expert but I believe it is the case neck that seals
 

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I sorry, I was trying simplify why the cases hold more heat.
That said, the case is being extracted before it can fully contract or ambiant air flow back into the case. That is one reason we have to use full length sizing dies and not neck size only.

Glenn
 

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I thought that brass started to contract toward original size before extraction/ejection started. The brass expands to completely seal the chamber, preventing gas blowback, and potential injury to the shooter. The hot gasses go through the port, and the timing on this action allows the brass to start contraction; allowing easy ejection. Correct timing prevents a torn cartridge rim, or head separation.

Early M16's had this problem when authorities sought to increase the cyclical rate of the M16, using hotter powder.

The M1A/M14 chamber is a bit roomier, but not to the point that the brass doesn't expand to fill it.
I dont think a case from an M14 contracts back to normal "as much" because extraction is started while pressures are still up. It does start to contract, but not as far as it would from a bolt gun, for the reasons you stated. I think it is the neck walls that really do the sealing of the gasses, anything behind that is supplemental...So if a case is not fully contracted, unlike in a bolt gun, it is also encountering some friction from both extraction and twisting, on top of the reasons you mention, but I think the proposition that the chamber and barrel acting as a heat sink on bolt guns is probably the main reason why bolt gun brass is cooler. If you go through a bunch of rounds on a bolt gun enough to get the barrel scorching, the brass does start coming out hotter, but bolt actions and shooting them is, in general, a much slower operation so heat build up in the weapon is generally less, plus extraction is so much slower. I "think" the M14 probably has one of the fastest, if not fastest, cycling of any gas operated gun. That's my opinion anyway, so there's lots of places for heat to build up and quickly....
 

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. I think it is the neck walls that really do the sealing of the gasses, anything behind that is sublemental, bolt actions and shooting them is, in general, a much slower operation so heat build up in the weapon is generally less, plus extraction is so much slower. I "think" the M14 probably has one of the fastest, if not fastest, cycling of any gas operated gun. That's my opinion anyway, so there's lots of places for heat to build up and quickly....
Absolutely Correct!BANANA1
 
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