Every choice is a trade-off. “You want armor to be light,
effective, and cheap. Pick any two.” So, sometimes you have to figure out what
are the most critical limiting factors, and go from there.
An ideal gun is light weight, accurate, shoots flat, hits
hard, has little recoil and comfortable ergonomics, has long barrel life, is reliable,
is low maintenance, has inexpensive and light weight ammo, and is easy to
operate… Yaahhh…. Riiiight…..
Back to reality.
The bullet does the work – everything else is just delivery
system. So, to stop a person or other living target (or set off a boomer), the bullet needs enough energy when it hits to do the job. Launching
the bullet imparts the energy into the bullet, and that causes recoil, requires
a gun, etc. Generally speaking, the greater the muzzle energy, the more the
recoil, the more wear on the gun, the greater the cartridge weight required, the
higher the chamber pressure, the more difficulty there is in noise suppression,
etc. So, an ideal cartridge would have some maximum tolerable muzzle energy,
and a minimum retained energy out to some desirable range.
What should those three numbers be? It depends on the
application. For the moment, I’ll consider military rifle cartridges (and perhaps Boomershoot guns). Maybe a
future essay will consider other applications.
If you generate much more than about 2000 ft·lb
of ME, a lot of smaller or less experiences shooters may have a problem flinching
or bruising from significant use, unless well trained and given sufficient
practice. Also, at closer range most bullets with more than 2000 ft·lb
will just waste an increasing percentage of their energy beyond the target,
after full penetration, on the backstop. (For comparison, 2000 ft·lb
is a typical muzzle energy for a .243 Winchester). Much less than about 400 ft·lb
is getting into a very marginal area for stopping power, cover or body armor penetration,
etc. (around 400 ft·lb is a typical 9 mm or 45 ACP round ME). For most
shooters, anything beyond a thousand yards is problematic for all sorts of
reasons, but out to that range an argument can be made, especially in places
like Afghanistan or Iraq, or in farm country with large fields, where distances
are long.
Challenge Summary: Muzzle energy less than 2000 ft·lb, greatest possible retained energy at
1000 yards, preferably at least 400 ft·lb.
It’s easy to find cartridges with less than 2000 ft·lb
muzzle energy. The problem is that most of them in larger calibers (30 cal and
up) are relatively fat, light, low BC bullets, or slow heavy ones that have a
trajectory like a rainbow and a time-of-flight measured in cups of coffee. The
smaller calibers (like .223), bullets are too light to carry much energy for
the distance, and start having severe wind problems at significant ranges. (For
comparison, a 5.56 NATO 77 gr bullet has a bit less than 1400 ft·lb
ME, and a 7.62 NATO 175 gr bullet has about 2600 ft·lb ME.)
It’s also easy to find cartridges that retain at least 400
ft·lb
at 1000 yd: just GO BIG. Heavy bullet, big brass, lots of powder, good to go.
But that generates more recoil, higher pressures, needs heavier guns, has
heavier ammo, more recoil, shorter barrel life, and so-forth.
Retaining energy argues that only high ballistic coefficient
bullets will likely manage to meet this challenge. A 6.5mm mid-weight bullet
with a high BC, like a Lapua 123 gr Scenar (BC of .547) launched at moderate
velocities, can be loaded to have both a ME less than 2000 ft·lb,
and have more than 400 ft·lb
at 1000 yards. One of the few current cartridges that meet this challenge is
the 6.5mm Grendel. It still has 372 ft·lb at 1000 meters in a factory loading, shot from a mid-length barrel. For
comparison, at 1000 yards, a 5.56 NATO 77 gr bullet has less than 200 ft·lb
of energy (similar to a .32 Auto), and a 7.62 NATO 175 gr has retained a bit
under 600 ft·lb
energy (similar to a typical 40 S&W shot from a 5” barrel). Also note that for reliable boomer detonation, a velocity of at least 1500 fps is generally required, and a typical 6.5 Grendel round is still moving faster than that at 700 yards (unless you are using a fairly short barrel).
The 6.5 mm cartridges have an excellent reputation with
hunters, as well as target shooters, and smokeless powder 6.5mm cartridges have
been around for well over a century, so there are a wide range of bullets
available for loading your own for any particular application you might have.
Ponder, think, consider, contemplate….
