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rox wrote:

zzr1100 wrote:At the last outdoor shot a fellow club member told me that I should make sure my ammo was kept in the shade,
Now I understand that ammo temp is important to consistency ..but exactly what difference does hot ammo make? Increased pressures, higher velocity ???

It can definitely lead to higher pressures.

As a pertinent example, the entire production of ammunition for the Imperial meeting had to be withdrawn because, despite prior testing, the conditions experienced this week lead to very large numbers of blown primers. Doubtless there are other causes forming part of the equation, but it seems likely that the high temperature was the straw that broke the camels back.

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Indeed. All the RWS 2013 batch was swapped out for 2012 after the large number of popped primers and ringed boltfaces it caused. Temperatures, plus a "more vivacious" powder than in previous years, seem to have been the main causes.

I kept all my ammo religiously in the shade, using a cloth to keep it covered on the firing point. Didn't have any problems myself, though I saw a good few people who left it out in the sun popping primers.

QuickLoad tells me a .308 round built to 2.890" with a 155 Scenar and a fairly hot load of N540 through a 24" barrel gives the following pressures:

50F/10C, 58,777 PSI, 2862fps
100/37C, 64,508PSI, 2916fps

So a potential rise in pressure of 5731 PSI and a rise in MV of 54fps.

The 50F/10C - 100F37C range show what could be seen for ammunition shot on a cool overcast UK day followed by the same ammunition left out in direct sunlight on a summers day.

Mine was kept in a box sandwiched between 2 ice-cold water bottles under a white towel. Still had gas leaks and a couple of blown primers before the switch - it certainly did not need to be left out in the sun to experience the problem. Case brass composition was being mooted by those I spoke to who were most likely to be in the know.

Would be interesting to see what Quickload makes of citeria closer to real-world, e.g. a 540 load to give 2950 fps from a 1-13 30" barrel at 15C, compared to the same at 30C.

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rox wrote:Mine was kept in a box sandwiched between 2 ice-cold water bottles under a white towel. Still had gas leaks and a couple of blown primers before the switch - it certainly did not need to be left out in the sun to experience the problem. Case brass composition was being mooted by those I spoke to who were most likely to be in the know.

Despite keeping my ammo shaded at all times I still had one blown primer during the Corporation, although this was actually with the 2012 PG60 batch rather than the 2013 batch, which gave exceptional accuracy in my rifle.

Anecdotal evidence from one or two people I spoke to suggested that softer brass may be the issue. For example, one chap I spoke to shot the same stuff in both rifles: the one with the tight chamber shot it fine but the one with a looser chamber blew quite a few. Not proof by any means, but is suggestive.

Cheers,

Gaz

rox wrote:Would be interesting to see what Quickload makes of citeria closer to real-world, e.g. a 540 load to give 2950 fps from a 1-13 30" barrel at 15C, compared to the same at 30C.

Quickload suggests a charge of 46.52 grains of N540 at 15C would give 2910fps through a 30" barrel with a pressure of 53,741 PSI. Increasing the temperature to 30C gives 56,552 PSI and 2,939 fps and going to 40C is 58,480 PSI at 2,957 fps. All are within the SAAMI Pmax (MAP) of 62,000 PSI and the CIP of 60,191 PSI.

Today is going to be warm, has anyone got a dummy round they can stand on the windowsill to see how warm it gets in direct sunlight?

A rather rough and ready rule of paw is that any metal object will be 20 C higher in temp in direct sunlight than the prevailing ambient, and that dull and rough objects will be a degree or two warmer than smooth and shiny ones. In Ganniland the REME didn't as a rule work on the panzers between 10:00 and 16:00 as they could actually get burnt from contact with the matt drab steel armour and wearing overalls and gloves just gave them heatstroke instead. They worked a lot of very long nights instead...

*Has 'orrible flashbacks of being roped in for midnight track bash parties*

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.

Is it just the powder or all the parts that effect the round?

I friend who lives in Utah said they all use varget powder as it is the most consistent with temp/humidity changes

ovenpaa wrote: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?

paxtond wrote:I friend who lives in Utah said they all use varget powder as it is the most consistent with temp/humidity changes

Hodgdon seem to have tried to this with their "Extreme" powders.