Dear Evil Engineer: Could I arm my snipers with ice bullets?

Dear Evil Engineer,

I manage a fast-growing agency which provides innovative sniping solutions to clients across a range of sectors. I like to work closely with mercenary snipers to ensure that they have access to the best tools and training available. Recently, I have been considering alternatives to conventional lead bullets. I’m fascinated by the idea of ice bullets which melt after use, leaving no trace behind them. Is such a thing possible?

Yours,
A stealthy villain

Dear villain,
Thank you for writing in. Despite an intriguing concept, I’m sorry to say that bullets made of ice may be some of the least effective tools imaginable. That is not to say that ice projectiles are entirely hopeless. Permit me to explain.

A bullet made of ice is highly unlikely to reach a sniper’s target, being prone to melting before so much as leaving the barrel. Firearms are quite inefficient, with only a small fraction of the initial chemical energy being converted into kinetic energy of the projectile – most of the remainder goes into heating various elements of the system, such as the barrel. Let’s consider some rough numbers.

A typical 180 grain (11.7g) lead bullet would have a volume of around 1cm³. How much energy would be needed to melt a bullet of the same volume made of ice and stored in a home freezer at -18°C? The specific heat capacity of ice is 2.093J/g/K, its density is 0.917g/cm³ and its latent heat of melting is 334J/g. So, 2.093x0.917x18 + 0.917x334 = 341J of energy, which would be sufficient to melt an ice bullet completely.

Now, let’s see how that compares to the thermal energy transferred to a bullet in a firearm. Ammunition cartridge manufacturer Hornady reported that the tip of a typical bullet leaving the barrel at 3,000fps (910m/s) is around 850°F (450°C). Other sources estimate it at closer to 500-550°F (260-290°C) for comparable muzzle velocity. Taking these figures as our lower and upper bounds, we can estimate that – based on ambient temperature of 15°C and specific heat of 0.129J/g/K – the thermal energy transferred to a typical lead bullet is in the range of 370-656J. Although there are other factors at play, such as a significant difference in thermal conductivity, we can say that an ice bullet will probably be at least partially melted upon pulling the trigger.

This matches what was observed on an episode of the Discovery Channel’s ‘MythBusters’. Experiments showed that ice bullets did not just melt but evaporate the moment the trigger was pulled. This was true even when the bullets were made from slow-frozen ice, which contains fewer air bubbles.

Of course, heating upon ignition is merely the first stage of heating our ice bullet would experience. Following that would be heating due to the friction as it hurtles down the barrel, then heating due to drag and shock wave effects, which appear at supersonic speeds.
There are other problems with ice bullets. Ice is very fragile compared with the materials typically used to make bullets, and likely to shatter before leaving the barrel even if it does not melt. Ice bullets would be impractical to use in many parts of the world, even if your snipers have access to mobile freezers.

Ice has low density, so carries little kinetic energy; an ice bullet carries approximately 11 times less kinetic energy than an equivalent lead bullet. This would also result in a low ballistic coefficient (ability to overcome air resistance in flight), a property which would be compromised further if the bullet was part-melted before leaving the barrel, losing its aerodynamic shape. A YouTuber who experimented with ice bullets with mixed results found that their low density resulted in significant curving in flight and very limited accuracy, even at 50ft (15m).

Ice is, unsurprisingly, a poor material for a bullet. With all these shortcomings in mind, what adjustments could we introduce to make the ice bullet fly?

First, the firearm could be replaced with an airgun (pneumatic acceleration) or even a rail gun (electromagnetic acceleration) in combination with a metallic sabot or casing. Use a smoothbore to reduce heating due to friction as the bullet travels down the barrel, and consider introducing an insulating sabot. You could also improve the properties of the ice itself. This could involve mixing the ice with 14 per cent sawdust or wood pulp to create ‘pykrete’: a material with almost concrete-like strength and toughness when solid, and low thermal conductivity.

Whatever gun you use, you will need to do some experimenting with reducing muzzle energy until you find the highest energy which permits you to fire a bullet without melting it. This will limit how much damage the bullet can do, and also range – less than ideal for a professional sniper.

All this being considered, an ice projectile would be most effective when working with limited muzzle energy and range, and instead maximising volume – something closer to an ice cannonball than an ice bullet. Such a projectile would be less likely to melt before reaching the target, would permit higher accuracy, and carry more kinetic energy. Although such a projectile would not penetrate the skin, it may deliver enough energy to cause blunt force trauma. Two different YouTube channels, in fact, have already demonstrated that it is possible to fire a large ice projectile from a custom spud-gun and shatter paving slabs or even dent microwaves at short range.
It’s hardly sniping, but it’s the closest you are likely to get to using ice for projectiles.

Yours,
The Evil Engineer

PS: An icicle dagger is an alternative (perhaps stealthier) weapon to consider – icicles falling from roofs have been reported to kill people.