Ammunition in the sunshine

[Gaz]

I placed a couple of dummy .308 rounds out in the sun yesterday and they got to 46.6C which would push the MV to over 3,000fps and pressure just over SAAMI max.

Which then opens up the question, how long does a cartridge have to have the bolt closed, during a string, for a "hot" chamber to raise the temperature to the point where pressure exceeds the specs?

Depends how hot the chamber gets, which in turn depends on the rate of fire... maybe there's scope for time/pressure/temperature experiments here?

[ovenpaa]

I know that when string shooting on a warm day with the Vikings S&L that once the round is chambered it is critical to get the shot off as quickly as quickly as possible or the POI rises quite significantly, the barrel can get very warm so the chamber must be cooking inside, in excess of 50-60C is my guess.

[Dougan]

I shot single snaps (15 rounds in a couple of minutes), double snaps ( 20 in a couple of minutes), and rapid (15 in a minute) with my SMLE at the Historic Arms Meeting - it was the warmest conditions I've shot the rifle in, and due to the temperature it wasn't cooling down much between details, and I was quite concerned about how hot the chamber was getting...

...how hot is radiator when it's too hot to touch?...as it was hotter than that!...fortunately the rounds don't stay in the chamber more than a few seconds with these courses of fire, and I deliberately load low (39 gn of N140) to be careful, and have never had a problem, but this time the bolt got a bit 'sticky' towards the end of the last detail, and I could tell the rifle wasn't liking it.

[EagerNoSkill]

At the 2013 Imperial it appears that the latest batch had distinctly different brass quality to the earlier batches.
The newer brass did NOT seem to have the consistency and intrinsic ability to withstand the pressure brought about by the increased heat.
Hence the volume of blown primers.
When reverted to 2012 cartridges ... problem went away and the the heat was still there - if anything week 2 was warmer than week 1.

[ovenpaa]

Dougan, our radiators have a surface temperature when on of around 55-60C (From memory)

[Matt]

On the radiator temperature..... All depends on what temperature you have the boiler thermostat set to and if you've got thermostatic radiator valves or low surface temperature radiators etc.

As a general if you've got your boiler stat on its highest setting the temperature of the flow pipe away from the boiler is about 80 degrees, take a few degrees away for heat loss in the pipe work going to the rads and your about there

[rox]

When reverted to 2012 cartridges ... problem went away and the the heat was still there

It reduced, but didn't completely go away.

At the 2013 Imperial it appears that the latest batch had distinctly different brass quality to the earlier batches.

It has been suggested that the brass softening may have started in 2012.

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