Advanced Marksmanship ammo temp?

[Outlaw45]

I've been reading the book, "50 caliber Sniper Course, Hard Target Interdiction." It has good info on shooting other than 50 caliber guns.

It talks about ammo temp a few times and how it can affect the ballistics. For example it says the air temp is 75 degrees and the ammo temp is like 100 degrees. What does this mean? How do you measure ammo temp? Is this because the rounds are in your pocket and get warmer? I'm confused? I can understand that a warmer(in temp) round could have more velocity than a cooler round. I think?

Can someone explain this to me?

 

[BOLTRIPPER]

Re: ammo temp?

it has been determined post publication that warm ammo by several methods aquire better consistancy....some allow the heat of the sun to warm....others may place ammo under a car heater.....others warm by keeping it close to body temp....and still others chamber in a hot rifle....SD's shrink....

i know that some that are published here say not to heat up ammo.....but the worm has turned for many, and have changed their mantra......don't believe....break out the chrono and see for yourself.

 

[Greg Langelius *]

Re: ammo temp?

Well, I've said so; but I'm also always interested in new info. If you could expand on the concept, maybe give a why and how or two, I'd be appreciative.

I'm serious.

I will contuinue to suggest that deliberately warming up ammo that's already near or past max charge may not be the wisest thing...

Greg

 

[Lindy]

Re: ammo temp?

Temperature variations cause variations in muzzle velocity, which will be accompanied by variations in ballistic performance of the bullet.

I have measured variations in muzzle velocity which range from 0.6 fps per degree F. to 5 fps per degree F.

The only way to know how much a given load will vary in muzzle velocity is to test it over a chronograph at different temperatures, bearing in mind that both powder performance and primer performance will affect the results.

Testing variations in grouping ability with temperature, especially at longer distances, is more difficult.

 

[Greg Langelius *]

Re: ammo temp?

This much I already understand. As I was taught, the energy content inherent in a propellant charge includes the heat energy the propellant possesses at the time of ignition. Warmer propellant starts higher, ends higher, hence more pressure.

In principle, a constant propellent temperature should derive a more stable velocity; but this breaks down if the cartidge resides in a hot chamber for any significant period. The cartridge brass is a very effective heat conductor, and can raise the propellent temperature more than just a little, maybe even enough to open groups up.

I do know that when I extract an unfired round that's been sitting in a hot chamber significantly longer than it takes to square up and fire, and replace it with a cool round, I shoot better. The hot round is left to cool for later in the string.

So maybe I'm doing the same thing, tryng to keep the propellent as close to ambient as I can.

Greg

 

[marduk185]

Re: ammo temp?

i generally try to fire all my rounds from cold bore at ambient, say 5 minutes between shots that way they should be same poi as what id shoot first round cold in a field setting. sop guess thats sorta what yall are doing but keeping everything warm.

 

[marduk185]

Re: ammo temp?

related question yall, what temp is required to "cook off" a round and will a hot vehicle ever approach this?

 

[Greg Langelius *]

Re: ammo temp?

Don't know, but it's hotter than a hot vehicle will produce. Not good for the ammo, though.

 

[308sako]

Re: ammo temp?

I

 

[Greg Langelius *]

Re: ammo temp?

When I shot in the desert (Raton NM), I too used a cooler. But I ran into a snag. If the ammo became wet from condensation, I blew primers anyway. Cold or hot, either way, please make sure the ammo and chamber stay dry.

Greg

 

[308sako]

Re: ammo temp?

Thanks Greg, I learned that awhile back, but humidity runs in the single digits here, though water still does not compress!

 

[Greg Langelius *]

Re: ammo temp?

Just tryin' to help...

 

[1SMALLJOHNSON]

Re: ammo temp?

Cookoff usually occurs ~ 350 to 450 degrees. Propellant, being largely nitrated cellulose, has an autoignition temperature similar to paper (cellulose). Remember the book "Farenheit 451"? That's the autoignition point for paper. If you keep your rounds below 350, they'll not autoignite. However, the propellant will rapidly degrade at temps above 180F.

My testing inside a vehicle, with loaded AR-15 mag on the dashboard, a hot sunny day in beautiful FLA. With a non-contact thermometer, I measured 165 F. That's too hot to touch, but it ain't gonna make the rounds go off. Pressure when shot? Yes, it'll be high. Will it be near proof? Not likely, unless you're using some "off-shore" powder....

 

[TCA4570]

Re: ammo temp?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">Temperature variations cause variations in muzzle velocity, which will be accompanied by variations in ballistic performance of the bullet.

I have measured variations in muzzle velocity which range from 0.6 fps per degree F. to 5 fps per degree F.

The only way to know how much a given load will vary in muzzle velocity is to test it over a chronograph at different temperatures, bearing in mind that both powder performance and primer performance will affect the results.

Testing variations in grouping ability with temperature, especially at longer distances, is more difficult.
</div></div>


This little of a temperature variation, could also be caused by passing clouds at longer ranges, right?

 

[Lindy]

Re: ammo temp?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">This little of a temperature variation, could also be caused by passing clouds at longer ranges, right?</div></div>

I don't think I understand your question. You didn't specify how much of a temperature variation you're talking about.

If you want to understand how much a given variation in muzzle velocity can affect point of impact at long distances, change the muzzle velocity by that amount in a ballistics program.