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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 ???

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As a pertinent example, the entire production of ammunition for the Imperial meeting had to be withdrawn because, despite prior testing, the conditions experienced this week lead to very large numbers of blown primers.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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Arrhenius showed that a 10 C increase in temperature can double the rate of a chemical reaction, thus ammo that is designed to perform at its peak at 20 C will reach peak pressure twice as fast on a day when the ambient temperature is 30 C. Also, please bear in mind that ammo left lying in the sun will be hotter than the reported ambient: the Met Office take temperature readings in a shaded box 3 feet above ground level.

Wow .., just thought about the implications of the heat of afghan on ammo !!

That's why military ammo is loaded to rather tamer specs than the SAAMI max... Compare 7.62x51 with .308, or 5.56 with .223

It has to cope with being stored in the UK, transported to Norway, forwarded to Belize and then finally used to put down a few Ganniwogs. The powder is given plenty of headroom to breathe and build up its pressure...

I was RCO today and briefed the shooters on the importance of keeping ammunition out of direct sunlight, as it was the day remained cloudy with temperatures net exceeding 24C. I had deliberately dropped my load down for the day as I expected things to be warmer.

I found this out the painfull way while pattern testing my Benelli M2 on Thursday as the 36g buckshot rounds were kicking like a mule, the recoil was much harder than normal, as was the 32g slugs too, so I've now got very painfull bruised shoulders and have a slug comp tomorrow, oh what fun that will be not, I hope the temps drop a bit otherwise I don't see me completing the full comp.

meles meles wrote:That's why military ammo is loaded to rather tamer specs than the SAAMI max... Compare 7.62x51 with .308, or 5.56 with .223

It has to cope with being stored in the UK, transported to Norway, forwarded to Belize and then finally used to put down a few Ganniwogs. The powder is given plenty of headroom to breathe and build up its pressure...

It's true for 7.62/308 but 5.56x45 is loaded to higher pressures than 223 Rem.

http://www.gundigest.com/ammunition-rev ... -vs-5-56/3

But the problem isn’t just pressure. That CIP pressure of 62,000 PSI? It is measured in a 5.56 chamber. If we take the same round, which shows 60,000 PSI/SAAMI (still 5,000 PSI over the .223 max) and put it into a .223 chamber, things get ugly. Really ugly, and really quickly. The pressure spike piles onto an already over-pressure round.

Edit: I had another link to the pressure difference but the Forum didn't like it!

So leaving 5.56 NATO ammo in the sun and then shooting it through a .223 wouldn't really be the best idea in the world !! Lol

That's why it's not a good idea to do load development and testing during the winter, the summer performance may be more that you expect. Mind you we've been having warmish winters and cool summers in the recent past.