SemiAuto Barrel Overheating?
 

I'm told that the average rifle barrel will heat up quite quickly under sustained semi-automatic firing, and accuracy suffers after as few as 5-10 shots. Is this true? And if so, how do chrome-lined barrels and such stand up?

yes, it is true.

Heat causes the barrel to deform, hence affecting accuracy.

Chrome lining is used to reduce wear on the barrel, not to dissappate heat.

But I personally don't know if the two are ever an issue, since the sustained fire and accurate fire are usually not done at the same time.

When the barrel starts to glow, it's time to let it cool off for awhile. :)

All barrels will heat up from only one shot.

It's not permanent though, is it?

Here is the semi-auto rate of fire information for the M14-type rifle:
(from FM 23-8)
1 min --- 40 rounds per minute
2 min --- 40 rpm
5 min --- 30 rpm
10 min --- 20 rpm
15 min --- 20 rpm
20 min --- 20 rpm
30 min or more --- 15 rpm

So, in 10 minutes, I could shoot 200 rounds without damaging the rifle. At the end of that 10 minutes, the barrel could easily give you a nasty burn. The accuracy would suffer, but it would still be within specifications. If you were shooting for match purposes, the accuracy would be off more than acceptable. Unless you really, really, heated up a barrel, or did something to it while it was really hot, you won't damage the barrel and it will retun to its "cold" position, or close to it.

There are several ways to reduce heating and/or maintain accuracy. You could flute the barrel, giving you more surface area to disipate heat. You could free-float the barrel, so that nothing is touching it as you shoot. You could cryo-treat the barrel, which removes internal stresses in the metal, so the barrel won't deflect as much when it heats. The effectiveness and cost effectiveness of all of these could be argued.

yeah, I typically stop when I can't hold a good site picture because of the 'mirage' from all the heat distorting my sight picture :p



and I thought barrels were chrome lined so as to prevent corrosion and slow down fouling of the bore?

Automatic weapons are employed with this concept in mind:

As the rounds are fired, the barrel heats up and starts to warp. This creates a "Cone of Fire" that is used to create a "Beaten Zone". Not a problem.

When most stuff is heated it expands and when it is cooled it contract. It should change about the same way each time from cold to hot. Then you can figure out about were you are going to hit.
I would think the thing to do is shoot around 5 times then zero in the scope. Let the gun cool off then shoot once to see where it hits. Each shot after should be off less as the barrel heats up.

Yeah, I'm an idiot. :P