Monday, February 10, 2014

Silver Ammunition Hardness

A very common trope in movies about werewolves and vampires is for an individual to grab a handy piece of jewelry or silverware and melt it down into a bullet mold. They then load the bullet and defeat the rampaging beast which is terrorizing them and their community. While this makes for an amusing piece of McGyvering it leads to the spreading of misinformation that may cost an individual their life in the real world.

The hardness of a regular bullet has a large range which helps determine its performance. When the pressure wave of the expanding gas hits the bullet it will obdurate, the official name for its expansion into the rifling grooves. If the bullet is too soft it will allow some of the gas to escape past it, reducing the energy imparted to the bullet. If the bullet is too hard it will fail to engage the rifling properly, this will also cause the gas to escape.

As the heated gas rushes past the bullet part of it will melt. The melted lead will be left on the barrel and lead to inaccuracy and partial blockage of future rounds. This is known as leading and is very detrimental to the weapon’s performance.

Pure lead has a hardness of 5 BHN (Brinell Hardness Numbers), original smoothbore and muzzle loading rifles were made with rounds anywhere from 5 to 10 BHN, handgun rounds may be anywhere from 10 to 18 BHN range, rifle rounds made from Linotype will generally be about 22 BHN. The hardest round used in rifles is 30 BHN.

There are two methods that may be used to determine the hardness needed for any bullet. They correlate to pressure and speed.

The ideal hardness for a round based off the pressure in pounds per square inch is:

While the maximum hardness a bullet should ever have for that pressure is determined as:
Max BHN=Pressure/1,422
This can be seen as the do not exceed number, it is the highest BHN a bullet can have and still properly obdurate.

The .45 ACP is rated to 21,000 PSI.
This means that the ideal BHN for the .45 ACP round is 11 while the do not exceed number is 15.
The 9mm parabellum has a max pressure of 37,500 psi this gives it a much wider BHN rating of 19.5-26 BHN

The Velocity rule is even simpler.
800-1000 fps 10-12 BHN
1000-1400 fps 12-16 BHN
1400-1700 fps 14-20 BHN
>1700 fps 20-30BHN

Using this chart a .45ACP will fall into the range of 850-1,250 fps based on the bullet weight (hollow point vs ball) and powder charge. Using this as reference this gives the BHN range of the .45ACP to be between ~10.5-14.5 BHN. This closely correlates to the Pressure rule and allows the loader to base the hardness off of the speed of the round for greatest effect.

The 9mm parabellum’s velocity falls into the range of 1,000-1,450 fps giving it a range of 12-16 BHN. This is obviously a much lower number than the pressure rating, but it is also the more accurate one and the range that a ammunition smith would ideally use.

If there is a disagreement between the pressure and fps chart the fps chart should take priority as it will be the more conservative estimate.
These numbers bring the issue with silver bullets to light. Silver is considerably harder than lead. Elemental Silver has a hardness of 24.5 BHN as opposed to lead’s 5 BHN. This makes it an option for most rifles and some handguns. Through the process of annealing the silver can be reduced to as low as 20.5 BHN. Unfortunately for the average movie hero/heroin, Elemental silver is too soft for the majority of uses such as eating utensils and jewelry. As a result other elements are added in to increase the hardness. Sterling silver can end up with hardness anywhere between 100 and 185 BHN. This means the minimum pressure that would be needed to force the bullet to obdurate can be anywhere between 142,200 to 263,070 psi. Even the highest rated +P rounds will only achieve 50-65,000 psi. This means that the absolute highest BHN a bullet can achieve and still obdurate is 45 BHN.

As a result any attempt to shoot a bullet such as this would result in the bullet either jamming in the gun or exiting with very little velocity. (Editor’s Note: In actuality the bullet will most likely hurtle out of the gun without any rifling benefit as silver has a much higher thermal expansion than lead so if you put a silver bullet into a normal bullet mold the bullet will be drastically undersized.)

For this purpose all bullets produced by HA use pure silver shot in anything but custom requested rounds (HA will produce up to 30BHN rounds by mixing precious copper with the holy silver).

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