Topic of the Month: April
2002
Some Comments about the
Temperature Effect on Ammunition
The initial cartridge temperature can
have a decided effect on the pressure (and associated velocity) generated inside a
cartridge case when fired. Fortunately, most factory ammunition is loaded conservatively
so that little danger will result even under some pretty extremes of temperatures. This
temperature effect was demonstrated in the National Match Ammunition of 1930. It seems the
ammunition had been exposed to the direct rays of the sun for several hours before the
match started. During the match several problems occurred because of excessive pressure.
The ammunition had to be withdrawn and replaced with ammunition stored at cooler
temperatures. It should be kept in mind that ammunition exposed to the direct rays of the
summer sun might reach temperatures much higher than that of the surrounding air.
Ammunition temperatures of 130 degrees F or more would not be unusual under such
conditions.
At the other extreme is ammunition exposed
to very cold temperatures for an extended period of time. Ammunition fired under these
conditions may develop inadequate ignition, resulting in low pressure and reduced
velocity. While these conditions are not dangerous, it could result in a miss or even a
wounded animal for the cold weather hunter. Fortunately, most modern factory ammunition
will withstand a pretty good swing in operating temperature before any detrimental effects
are noted.
Fig.1 Effect of temperature on pressure for
three military cartridges
The Frankford Arsenal and the Burnside
Laboratory performed temperature tests on ammunition in the 1930’s. It was found that
the velocity increase was about 1.7 fps per degree F. The standard temperature was taken
at 70 deg. F. Based on this number, ammunition heated from 70 deg. F to 130 deg. F would
experience an increase of about 102 fps increase in velocity.
In a similar study the U. S. Ordnance
Department determined the effect of cartridge temperature on pressure using ammunition
loaded with three types of military rifle powder. The results of this study are shown in
Figure1. Note that the cartridge loaded with du Pont 1489 powder showed an increase in
pressure of about 5,500 psi for the temperature range of 70 deg. F to 140 deg. F. This is
about a 11% increase in pressure and could cause problems in some rifles. Lesson …..
keep your ammunition out of the hot summer sun or confined spaces in the summer time.
In a related subject, what happens when
ammunition is exposed to very high temperatures generated by a fire? If you were a fan of
the old western movies, do you remember how a handful of cartridges, dumped into a
campfire by the bad guy, had everybody running for cover with cartridges exploding and
bullets whizzing around and ricocheting off rocks and nearby trees? Well, fortunately this
happened only in the movies. Tests performed by the Sporting Arms and Ammunition
Manufacturers Institute have shown that cartridges exposed to a fire will not explode
simultaneously, but rather piece by piece; that the bullets are not projected with any
great velocity; and that the material of which the cartridges are made of will usually fly
no more than a few feet. In one test both rifle and pistol cartridges were stood up in the
bottom of an electric lead melting pot. A piece of corrugated cardboard was placed over
the top of the pot and the heat was turned on. When the cartridges exploded in no case did
the bullets pierce the cardboard or even dent it deeply. While I don’t recommend
standing around a fire and watching a batch of ammunition cook off, the danger is much
less than most people have been led to believe.
Watch our web site for the next topic of
interest "Guns Fired Underwater." Until then, shoot safely and know where you
bullets are going.
Sincerely,
The Ballistician
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