I have recently been looking into subsonic rifle loads for full-sized bullets. By full-size I mean enough power to be considered adequate for hunting and defensive purposes. Less recoil and less noise are attractive, but I have found that sub-sonic rimfire (.22LR) can’t even clean-kill whistle pigs.
One way to go, of course, would be to just get a carbine that shoots pistol ammunition for the purpose. The popular handgun loads all come with subsonic options or, as in .45, never do go supersonic.
The speed of sound is about 1,130 feet per second. That varies a bit depending on conditions. I feel comfortable working with 1050 and below as subsonic and figure 1200 and above to be supersonic.
The loadings listed in the HANDGUN ENERGY box assume pistol-length barrels. Launched out of a 16-inch or 18-inch barrel, there will be greater speed. The 200-grain .40 above might go supersonic in a carbine barrel. Still, for comparison sakes, I wanted to show the speed and subsequent power using handgun cartridges to achieve subsonic velocities.
Projecting those energy levels out to 100 yards reduces the power of handgun ammo significantly. Their ballistic coefficient (aerodynamics) is not as good as that of rifle bullets.
A carbine using handgun ammo is not a bad choice for easily and conveniently having the reduced noise and recoil of subsonic ammunition while achieving defensive and light hunting capability, but it should not be confused with full-power guns.
However, a young upstart cartridge is getting a lot of attention in a big hurry. The 300 AAC Blackout is readily available from many rifle and cartridge manufacturers. Built on an AR-15 platform as a complete rifle or as an interchangeable upper on an already-owned lower, this gives a nice heavy-hitting short-range rifle that, at its lightest loading is handgun-carbine powerful, yet can compete with the AK-47 and 30-30 for hunting and defense in its higher-power cartridge loadings.
Best of all, the 300 AAC Blackout rifle on an AR platform can seamlessly switch from high velocity to subsonic by merely changing the ammunition you put in its magazines.
As far as the mechanics of the rifle are concerned, you could mix subs and supers in one magazine without any failures to feed and fire. Don’t do that, please. It would just be stupid to have some rounds going high and others low.
My point is that the rifle doesn’t know fast from slow, light from heavy, supersonic from subsonic. No changes or adjustments to its operation, or the magazines are needed.
I feel compelled to point out that the 100-yard zero for your scope or sights WILL BE DIFFERENT between your supersonic and subsonic loads. You will actually have to test, practice and document that on your range card. Put the required sight/scope settings on a card and tape it to your rifle if appropriate. The dual-purpose rifle is only useful in the hands of a rifleman.
The sights very definitely must be changed. As you can see from the chart, trajectories downrange are significantly different between light supersonic loads and heavy subsonic ones. The supersonic loads drop six inches between 100 and 200 yards while the big slow guys drop nearly a yard.
Slow is low. The difference is between a hit and a miss (at 200) … either way too high … or too dang low, depending on the mix of your sight adjustments and the load you are launching. You DO have to pay attention to what you are doing.
However, the power of the low, slow, subsonic 300 Blackout bullet 200 yards away from the rifle is smack-dab in the middle of handgun cartridge muzzle energy figures that are widely considered to be more than adequate. The fast movers are good at nearly anyone’s theory of what works for medium game or defensive purposes.
I’m finding a lot to like in the 300 AAC Blackout.
Real handgun cartridges all begin with a 4.
Variations on the above phrase abound in the gun culture.
Modern propellants and bullets have obsoleted it,
Not to mention the .357 magnum which predated the .40,
and beat all of the above for muzzle energy,
but it remains a snappy phrase.