I recently got a KAC QDSS NT4 for my SBR M4. It has a 14.5
barrel.

What 5.56 subsonic loads are out there for it?

If you'll accept that I'm being serious and not sarcastic, I'd suggest looking into a good .22LR conversion for it. Reason I say that is that unless you find a load with an extremely heavy bullet, that's what you're really ending up with anyway - a low-velocity, .22-caliber projectile that's weaker even than a normal .22LR round.

I've used one (a ciener) on a suppressed carbine for quite a while, and after the break-in period, it actually became my squirrel gun for a few years. It was surprisingly accurate and phenomenally reliable with remington subsonic HP's, and stunningly accurate but not particularly reliable with Aguila's heavy 60-grain SSS load. I was worried about lead buildup in my suppressor from using non-jacketed LR bullets, but AAC assured me that as long as I didn't run more than a couple hundred or so of them between sessions with full-power ammunition, that the pressure and heat from the full-power stuff would keep any buildup blown out.

If you must stick with subsonics in a full-size .223/5.56 cartridge case for whatever reason, I have no idea what to suggest; sorry.

Never tried it but http://www.cheaperthandirt.com/product/AMM-2124

Those will work but won't cycle the bolt so I would them.

Youre probably not going to find any that will cycle a standard gas set up.  Even the 100gr mag length OTMs don't develop enough pressure.

I know my load book shows low velocity loads with pistol powders. But I think they are still super sonic. I will take a look later since it's in the basement and see.

I know my load book shows low velocity loads with pistol powders. But I think they are still super sonic. I will take a look later since it's in the basement and see.

I would not recommend the 22 conversion kits.  If you are a decent shot, you will be very disappointed with the accuracy.  You are shooting a 40 grain bullet out of an oversize barrel with a 1:7 or 1:9 twist on a bullet that wants a 1:16 twist.  You will cut groups in half with a dedicated upper.  I don't notice much of a difference offhand.  However, I do notice a big difference prone, kneeling and sitting.

Also, a subsonic 556/223 load is just not going to work well.  You are trying to cut velocity by close to 70%.

Thanks guys. I'll stick to my usual 55gr FMJ

On the .22lr bullet bore-size issue, iirc, the bore sizes are .218 for .22LR and .219 for .223. Groove diameter (again iirc) is .222 and .224 respectively; so the difference is real but not particularly huge by any means.

On the appropriateness of twist rates, the 60-grain aguila sss mentioned above is right in the middle of the typical weight ranges the gun gets put thru it.


Don't get me wrong - I absolutely wouldn't use a .22 conversion kit as a substitute for .223 subsonics for defensive use (nor would I use .223 subsonics for defense in the first place); but for hunting or shooting plates, the .22 kit gives the same power at substantially less cost.