utm fk

38 G&A February 2020 | bullet boArd
insufficient gas volume
to run the gun. The other
approach was to use
heavier charges of very
slow propellants. This still
didn’t work because the
slow propellants didn’t burn
fast enough to produce
much more gas impulse to
the gas system. It just blew
a lot of unburned propellant
out the muzzle. They
did not produce any better
performance uniformity and
they fouled the gas system with carbon very rapidly.
The introduction of IMR (now Hodgdon) Trail Boss
propellant finally offered a propellant that filled the case,
produced uniform performance and subsonic velocities.
However, because of the low charge weights, it still didn’t
function a gas-auto firearm. If you have an urge to experiment
with your own subsonic loads, you won’t be able to
beat Trail Boss for performance as a single-shot round or
for a bolt action. Experiment with bullet weights to tune
your load for best performance.
Muzzle velocity uniformity is critical to the accuracy
performance of subsonic ammunition. A 100-fps muzzle
velocity variation, which is not uncommon in subsonic
ammunition, is nominally a 10 percent difference in muzzle
velocity. This would correspond to a 300-fps variation in a
3,000-fps cartridge. This muzzle velocity difference from
the slowest to fastest round results in substantial vertical
stringing on target.
With a subsonic round, vertical stringing can become
quite big at longer ranges. In subsonic ammunition, a 100-
fps muzzle velocity variation will result in a 31/-inch elevation
point of impact (POI) difference at 100 yards and 12
inches at 200 yards — and that’s not considering anything
else but muzzle velocity variation.
terminal Performance Subsonic ammunition has significant
limitations when it comes to terminal performance.
Because of the low velocity, it is difficult to obtain terminal
performance in anything other than a pistol caliber. Rifletype
bullets do not have the large frontal area that hollow
points and other pistol bullets do. Most of the platforms
such as the AR-15/M4/M16, M14 and SR-25 have a fixed
feed ramp configuration and do not
function well with a blunt bullet profile
and large meplat diameter. This puts
severe restrictions on what a bullet
designer can do to try to design a bullet
that will expand at subsonic velocities.
One very interesting aspect of subsonic
terminal performance is that once
a bullet drops below supersonic velocities
the drag on the bullet drops to low
DRAG COEFFICIENT
G7 DRAG COEFFICIENT
MACH NUMBER
HORNADY 190-GR. SUB-X
SUPERSONIC vs. SUBSONIC
VELOCITY LOSS
VELOCITY (FPS)
RANGE (YDS) SUPERSONIC SUBSONIC
0 2,500 1,050
100 2,305 998
200 2,119 956
300 1,942 919
FIGURE 2
FIGURE 1
levels and is, for the most
part, constant. This means a
subsonic projectile will lose
velocity very slowly. If a projectile
can be designed that
expands and has a couple
hundred fps velocity range
over which it expands, it
will give terminal performance
over a range that
is outside the ability to
accurately shoot it.
Look at the G7 drag
coefficient curve in Figure
1. As you can see, the drag of the projectile drops rapidly
after its speed drops below Mach 1. By .9 Mach, 1,005 fps,
nominally the muzzle velocity for a lot of subsonic ammunition,
the drag coefficient has dropped tremendously
and does not get a lot lower. As an example, look at the
retained velocity table in Figure 2 for Hornady’s new .300
Blackout 190-grain Sub-X bullet ($23, hornady.com). The
Sub-X bullet barely loses 130 fps at subsonic velocities in
300 yards. The same bullet at a supersonic muzzle velocity
of 2,500 fps loses 660 fps in 300 yards. The Sub-X bullet
will expand down to about 900 fps. It offers terminal performance
far beyond the effective range of the cartridge.
enter the 300 blackout The 300 Blackout is, for all practical
purposes, the .300 Whisper developed by J.D. Jones of
SSK Industries for use in Thompson/Center Contender
single-shots in the 1990s. It was based on the .221 Fireball
case necked up to 30 caliber and was aimed at efficiently
and reliably producing subsonic velocities with heavy
bullets. The cartridge nominally uses 10 grains of propellant
and provided an excellent round for adapting to the
AR platform for a dedicated, gas automatic, suppressed
firearm for reliable functioning subsonic performance.
With a short barrel, short gas tube and a suppressor, the
.300 Blackout in the AR-15 platform performs reliably. I
have to add at this point that to get truly reliable subsonic
performance from an AR chambered in .300 Blackout,
a suppressor is highly recommended. The very limited
amount of gas available from this cartridge benefits substantially
from a suppressor’s added back pressure to force
more gas back into the system.
The .300 Blackout has been a highly successful cartridge
that has also been responsible for the
current interest in subsonic ammunition,
and the rapid increase in demand for
suppressors. There are other offerings
available for subsonic ammunition. Let’s
take a look at a few.
other Subsonic Ammunition Major
rimfire manufacturers offer subsonic
.22 LR ammunition including Aguila