# Subsonic 300 blackout load development



## Griffin

Started some load development yesterday and had some good success! We used 2 guns, both suppressed. Gun one is an 8" lefty stag arms 300 blackout with YHM suppressor 1:7 twist, gun 2 is Rine's I believe 16" scorpion 300 blackout 1:8 twist with same suppressor. 
Load development:
For gun 1- 150grn GMX with IMR 4227 powder
Started out with 11grns powder 2.25 COL. Avg velocity @ 15 yards 1158 fps
Drop to 10.8 grains resulted in 1123 fps
Drop to 10.3 grains resulted in 1094 fps
Finally ended at 10.0 grains and 1047 fps 
Perfect, shoots nice and cycles great too!

For gun 2- 200grn accubond with H110 powder
Started with suggested 7.3 grains. Resulted in 791fps
Increase to 8 grains resulted in 923fps
Increase to 8.5 grains resulted in 933fps (odd)
Final increase to 9.2 grains resulted in 1042fps
Shot great, amazing grouping, ejected cases...but didn't load the next round?
Decided to try the same load with the 4227 powder out of curiosity before packing everything up. 9.2 grains with 200 grain accubond resulted in 919fps but still didn't grab the next round.

Another strange problem with both guns is it wasn't consistently fully racking the rounds...might shorten up the rounds next time.


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## kanga69

I am by far no expert but started playing the game recently. We had similar problems and had to change the buffer spring. Good luck.


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## mrvmax

When I developed my loads the subsonic was easiest to work up. The supersonic are more difficult for me due to the shorter bullets. I have three different mags i use depending upon the round (and all three have different followers). My steel GI style mags seem to work better for subsonic and the PMags for the supersonic. It does take some experimenting to get it working, my main problems were feeding issues due to mags. I never had to do anything to the buffer and my lower is a cheapo New Frontier Armory polymer lower.


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## nasakid

Try some A1680 powder. I started at 11.2gr for 220SMKs, but I need to chrony it and work it down. I had to switch to the Nemo buffer spring to get mine to run without a suppressor.


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## Bird

My inclination is a mechanical one rather than needing more pressure in the cartridge. Make sure the follower and spring in the magazine moves without any restriction, is clean and well lubed. Consider swapping buffer springs. Ensure gas block tube on gun 2 is clean and doesn't have anything clogging it. Sub rounds tend to be a little dirtier than full power rounds since you don't have all the flaming heat of a super round burning everything out every shot.


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## Griffin

All sounds like good advise to me. Gun 1 is mine, hoping to get a new stock and spring soon, the buffer spring is the loudest part of each shot. A good thorough cleaning and some internal polishing seems to have my problems solved. 
Gun 2 is brand new, I think some a few good cleanings and some more shooting it'll come around. 
Bird- these powders seem to burn a lot cleaner than the trailboss! Wish it wasn't so dense and would work for the 308 subs!


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## mrvmax

All the AR's I have ever shot or owned like to be lubed well. Make sure the BCG and other internals have plenty of lubrication to ensure good cycling.


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## BradV

The 200gr / h110 load would not generally be regarded as an optimal subsonic load in .300 BLK.

Accurate 1680 (if you can find some) produces more gas volume than H110 and will give you more pressure at the gas port to cycle the action while still being able to keep your rounds subsonic. H110 works excellent for supersonic loadings.

Here is a bit of reading on some results (not mine) using A1680
http://www.300blktalk.com/forum/viewtopic.php?f=141&t=84221

Now if 1680 does not cycle, it is definitely possible that the gas port of that particular barrel is either not aligned with the gas block, or is slightly too small. Carbine gas systems on a .300 BLK can be on the edge of cycling without a can, but normally with a can there aren't any issues, given the gas port is aligned, sized correctly, and everything else is operating as it should.

My go to load in my rifle is 10.5gr A1680 with a 208gr Hornady A-max seated to 2.20" OAL. This gives me ~1030 fps and holds the bolt open on the last round. CMMG 16" Carbine gas system, YHM can. Standard carbine buffer and spring.


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## Griffin

That makes since with the H110, I didn't like that powder at all, it's way too fine for my liking!


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