Monthly Archives: December 2020

A Tale of 3 Balls

THE MACRO:

Specific Gravity sounds like a pretty hard concept to understand…and it can be a little tricky, especially when it comes to applying it to small arms ballistics.  But it doesn’t have to be hard.  It’s really as simple as a Baseball, a Tennis Ball, and a Whiffle Ball… let me explain… 

Specific Gravity is the term that is used to define an objects mass as it relates to an equal volume of water.  For example:  Water has a specific gravity of 1.  Lead has a specific gravity of 11.34.  This means that 1 cup of lead is 11.34 times heavier than 1 cup of water.  Copper, another common bullet material, has a specific gravity of just under 9, or a cup of copper is 9 times heavier than a cup of water….which means that copper has roughly 80% mass/density of lead.  Copper/polymer and sintered bullets are typically around 6, or 53% the mass of lead.

In this day and age, too many shooters and hunters think no further than muzzle velocity, bullet weight and ballistic coefficient.  But that’s just the “MICRO”, we’ll elaborate on the MICRO later in the essay… But this mindset ASSUMES ALL projectiles to be on a level playing field, and that is simply inaccurate.  A very wise man once said, “the best bullets will always have lead in them” and I concur 100%. 

A number of years ago, I was solicited to help with a very “interesting” project in which a company was working on injection molded projectiles from various polymer/metal matrices.  Seriously wild stuff…and it wasn’t until this project that I truly began to understand just how important Specific Gravity is to small arms ballistics as a whole.  The entire firearms industry seems to have been inadvertently built to work best around a specific gravity of something in the neighborhood of 10 – from twist rates to slide weights…it’s a very real thing.  On this project, we were working with materials that had a specific gravity closer to 6, or just more than half the mass density of lead.  This led to all sorts of very challenging and complex learning opportunities from gyroscopic stabilization to firearm function, and how different density materials perform terminally.  It was a fantastic learning experience. 

In the current commercial firearms and ammunition space there are 3 predominant types of projectiles.  Lead core/copper jacketed bullets (aka: cup & core bullets), solid copper (or gilding metal) bullets, and IMM and/or sintered bullets.  There are others, but they all common types fall within these boundaries.

I’ve had many friends ask, “so, what’s the difference – really” especially when discussing a traditional cup & core bullet in comparison to solid copper hunting bullets.  After a lot of thinking, it finally occurred to me while playing Whiffle Ball with my son.

To more easily understand the role that specific gravity plays in ballistics:  Find a baseball, a tennis ball, and a whiffle ball.  Pick out something to throw the balls at that will react, and not get ruined in the process.  Throw each ball at the target as hard as you can throw it, being careful to put the same amount of energy into each throw.  Which ball causes the most effect on the target?  Next, find an open field and throw each ball as hard and far as you possibly can, being careful to use the same amount of effort and technique when throwing each ball.  Which ball traveled the furthest?  

The effect that specific gravity has on objects that fly should be clarifying very rapidly.  The baseball represents a cup & core bullet.  The tennis ball represents a solid copper or brass bullet, and the whiffle ball represents a copper/polymer or sintered bullet.

This is a gross oversimplification to illustrate a concept, but the concept is very real.  

“HOW can there a difference between two 30 caliber, 150 grain bullets that have the similar shape…but one is made of copper and the other is made of copper and lead?”  The answer is all around us – the atmosphere.  The atmosphere has weight and density – approximately 784 times less dense than water, but never-the-less IT IS dense enough to matter.  For an object to efficiently pass through the atmosphere, the more dense the object is, the less ability the atmosphere has to create resistance to slow the objects travel.   Back to the ball example, the whiffle ball is closest to the weight of the atmosphere…which means that the air is going to be able to slow the whiffle ball more quickly because the whiffle ball lacks the mass to store the kinetic energy that is being transferred to it by the throwers arm.  The baseball, having a higher specific gravity, may start a bit slower, but it is able to store more of the kinetic energy and overcome the atmosphere more efficiently…it will also overcome other targets that it may encounter more efficiently.  This is called “inertia”. 

Ballistics seems to be hard for a lot of folks to understand.  It’s really as simple as physics.  It is also important to keep in mind that there is not and never will be a “magic bullet”.  All bullets have things they excel at, and that also means that they’re not as good at something else.  There are instances where having an extremely low SG bullet is precisely what you need, as you don’t want that bullet to over penetrate or escape a shooting range and hit an unintended target a long distance away.  Low SG bullets will slow much more rapidly than their cup & core and copper cousins.  Certain locations require the use of non-lead bullets and it is great to have many excellent solid copper hunting bullets to choose from, however, one must keep in mind that they simply will not have the same long range capability as cup and core bullets, especially beyond 400-500 yards.  It’s great to live in a day and age where we are afforded so many great choices to achieve the most precise solution.

THE MICRO:

There are lots of other factors that go into how efficiently a projectile flies…but the projectile’s SG will always the lowest common denominator.  Other things that make a difference are the way the projectile is balanced in flight, the relationship of Center of Pressure and Center of Gravity, the bullets angle of attack in flight – the amount of the bullet’s surface that is presenting itself to the atmosphere and can be a function of trajectory or yaw – all these things will factor into how efficiently a bullet flies to it’s target.   The “gravity” of the atmosphere also makes a difference.  In the old days, trajectory tables had to be manually converted to account for approximate atmospheric density due to elevation or other “macro” atmospheric factors.  Today, ballistic “solvers” aka: computers do all the pencil work…that said, it’s always wise to “true” all the numbers on a specific load as while ballistic coefficient numbers and calculators are more accurate than ever, there is no substitute for real world performance…so don’t forget to pull yourself away from the computer screen and get some trigger time… 

An observation in passing:  The vast majority of ELR competitions are currently shot with 375 and 408 CheyTac (and similar) cartridges.  To this point in time, almost all, if not all, bullets for these cartridges are machined from brass.  While the cartridges are significant improvements in terms of “order of magnitude” their performance would receive an exponential gain if a bullet manufacturer were to make a very high precision cup and core style match bullet, as it would significantly increase the Specific Gravity of the bullet… food for thought…