Where subsonic ammo comes into play is taking this down even more and getting rid of the secondary supersonic crack from the bullet. A good suppressor can get that down to 135 dB, below the impact noise threshold the Navy beat into my head years ago from NAVOSH 5100.19. With a typical 16-inch barreled AR-15/M16 delivering a 167 dB sound pressure level at the muzzle (depending on environmental conditions) with M193 55-gr FMJBT round, we are already exceeding the 140 dB impact noise threshold for hearing conservation. While we may only be talking 2-3 dB improvement changing from supersonic to subsonic ammo, remember that the decibel scale is logarithmic, so every 3 dB change represents doubling or halving the energy of the reference. A typical “can” (slang for suppressor) may have a 32 dB rating (maybe even 38 or more if it has a wet rating as well, where the suppressor/silencer is designed to be oiled before shooting). It’s a segment of the shooting community that talks in dB (decibels) and how much average noise reduction they’re achieving. 223 or 5.56 ammo often comes from the NFA community-those shooters and collectors who are using a suppressor (although the term silencer is even used in the industry, it’s an inaccurate term for what it does) and are looking for that much more of an edge to be even quieter. ![]() Like many things ammo, there are almost as many opinions as there are varieties of ammo and the ammo that is apparently out there in the market is vaporware. Good luck if you test this load data and let me know how it works for you.I occasionally see requests for. The Alliant powder seemed a little quieter than the IMR powder. There is still a lot of gas coming out of the ejection port. These loads were not particularly quiet suppressed. I needed to load these rounds with a COL of 2.260”. Also, before you test any load shoot it on paper to make sure your rounds are not keyholding without a suppressor. You need the extra pressure the suppressor provides. Note, you must use a suppressor to get the bolt lock open. I suspect this load will produce somewhere around a 1.5 – 2 MOA grouping. I did a quick 4-shot test to take a look at the accuracy. ![]() Here’s the grouping of the IMR load from 25 yards. Note, you will probably have to tweak the load data to your specific firearm. This is the AR I used with a Mystic X suppressor. The standard Gas Hole Diameter of 0.070” works fine. You will need a 1:7 twist barrel at least 16” long with a carbine gas port. I was able to get both Alliant 2000-MR and IMR 4227 working. Outlaw State Bullets recommended a load using IMR 4227 for a cycling AR subsonic round. They make a hollow point bullet that will expand at low velocities. That was the only place I found that manufactured anything over 100 grains. 224 bullets from Outlaw State Bullets LLC. The powder in the Atomic load looked very similar to Alliant 2000-MR, so that’s where I started. The bullet is 112 grains and the primer looked like a Winchester small rifle primer. I took apart the cartridge and what I found was they are using 15.6 grains of a fine flattened ball powder. ![]() Obviously you need a heavy bullet for cycling an AR 223. The clue the Atomic ammo gave me was they figured out how to get enough pressure at low velocities to cycle the bolt. The Atomic sub ammo gave me around 1150 FPS from the 10.5” barrel and around 1500 FPS from the 16” barrel. The ammo cycled and locked the bolt open in both the 10.5” and 16” barrels. I could not get the Atomic subs to stabilize with my fast twist 1:7 10.5” or 16” 5.56 NATO barrels. I had high hopes for the sub ammo, but it didn’t work. I ended up purchasing some Atomic Tactical Cycling 5.56 NATO Subsonic Ammunition from Midway at $60 for a box of 50. Some of the load data posted that claimed to cycle an unmodified AR did not work for me. It seems a few others have tried this and ended up not having much luck either. I tried to find some load data but had no luck. 223 subsonic load working with a standard AR15 platform that cycles. For the past 7 months I have been trying to get a.
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