Rifles 101: How Bullets Kill
A pile of 6mm Hornady 109 ELD-Ms ready to go to work.
Rifles are Systems.
A hunting rifle is a tool that reliably places a projectile at velocity into an animal at distance in order to swiftly kill it. Everything in that sentence- the reliability, the projectile, the velocity, the animal, and the distance, is an input in the rifle system that determines how it should be built. In order to describe these considerations, the only taxonomy that makes any sense is to start from the animal and work backwards to the shooter’s shoulder.
The Projectile.
Projectiles kill by causing a short & wide “temporary stretch cavity” and long & narrow “permanent crush channel” within the animal that disrupt primarily its cardiovascular system, secondarily its skeletal system, and at times (thirdly) its central nervous system. Limiting the conversation to bullets (ignoring arrows, bolts, spears et al), there are two prevailing schools of thought.
The first is that a hunting round should be robust, creating a very long permanent crush channel at the expense of temporary stretch cavity size. Sometimes this is referred to as “penciling” by critics, and supporters mainly tout the flexibility of round placement as a benefit. For example, robust bullets can break down skeletal structure far deeper in the animal than alternate designs.
The second school of thought is that hunting rounds should cause as much blood loss as fast as possible via frangible or expansion effects, even if that means the majority of their mass does not penetrate as deeply as a robust round. These rounds tend to create a larger dispersion of copper and lead fragments in the area of meat around the impact path.
In general, we can categorize “monos” (primarily copper bullets) in the robust category and “match bullets” (generally thin-jacketed copper and lead projectiles) in the frangible category. Terminal effects can only be understood from data collected after animals have been engaged. Both kill animals.
One thing is true regardless of bullet type - the rifleman must understand the minimum impact velocity for his chosen projectile, and design his rifle system around it. If the minimum impact velocity for performance is 2200 fpm for a mono and 1800 fpm for a match bullet, for example, that means that the mono-designed rifle will either be range-restricted, require a longer barrel, or require more powder behind it in order to compete with the match bullet’s terminal performance.
Because the projectile encounters drag whilst flying through the air, its shape greatly impacts its retained velocity at distance and the amount it drops while traveling. This phenomenon is handily captured in a figure called Ballistic Coefficient or BC. The BC does not affect terminal ballistics, only external ballistics (the characteristics of flight while the projectile is moving through air). A high BC bullet will retain more velocity at range, drift less in wind, and drop less vertical height to the target. This is why a 6.5 creedmoor can have lower velocity than a .308 at the muzzle, for example, yet magically have a higher velocity 500 meters downrange.
The Cartridge.
Cartridge selection is deceptively simple. First, choose a projectile in whose terminal performance you’re confident. Gain that confidence by perusing hunters’ forums with many pictures of dead animals such as Rokslide. Take the manufacturer’s minimum impact velocity (“MIV”) number in fps and find evidence from prior engagements that this impact velocity indeed consistently yields results. Add a buffer of your choosing, such as 100 fps, and correlate this new “AMIV” (adjusted minimum impact velocity) using a ballistic calculator (like Hornady’s 4DOF app) with barrel lengths and cartridges which interest you.
For example, if you have chosen the 6.5mm 143 grain ELD-X projectile, you might explore with 4DOF what performance looks like in a 6.5 Creedmoor, 6.5 PRC, or 6.5 SAUM at various barrel lengths. At whatever distance 4DOF tells you the projectile drops below your AMIV, that’s your max distance for that cartridge. Is the distance insufficient for your hunting style? Increase the barrel length or cartridge powder load (like going from 6.5 creedmoor to PRC). Is the AMIV distance far beyond your hunting ranges? You’re probably taking too much barrel into the field, or handling too much unnecessary recoil. Cut the barrel down or downsize the cartridge or both.
The AMIV and the terminal performance of the bullet are the only factors which contribute to killing. Oft-touted figures like energy (measured in ft-lbs) or Killing Power Score (KPS) are now considered by professionals to be old wives’ tales with no objective evidence to back up their use as a guide. Secondary factors like sectional density are not important enough to figure into the hunting rifle building equation.