New to the 300BLK

[JeffWard]

I'm building a Pistol Lower (Sigtac Brace), and a 300 BLK Upper. I know there are issues with subsonic loads cycling. I've done some searching, but can't find a concise answer to my question.

For a Pistol Build (future suppressor planned, but UN-suppressed currently) COMPACT gun:

- What barrel length is best for reliable function both super/sub-sonic (without a can)?
- Why? and What factors effect this most?

I want to build a SHORT truck-gun, with a full-length hand-guard, forward projecting flash hider (Noveske/IROC/etc), with the flash hider tucked mostly inside the rail. For defensive use, and occasional feral hog hunting.

What's my best combo?

Thx,

FNG JeffWard

[buzziebells]

Hello and welcome.

This is the combination I use and I do not think I could be happier with any other set up.

For the barrel: AAC, 9", 1:7 twist, pistol length gas system.
Reason why: AAC developed the cartridge, therefore they should know best how to build a barrel for it, 9" length is more than long enough to burn all the powder in subsonic loads and long enough that you lose very little velocity with sonic loads, 1:7 twist to stabilize everything from light sonic to heavy subsonic loads, pistol gas for reliability in cycling all loads.

Handguard: Seekins MCSR V2 in either 12" or 15" length.
Reason why: MCSR will fit over almost all suppressors, therefore when you get the suppressor, you will already have a handguard that is compatible. MCSR will accept rail sections so that you can put the rails where you want them when you need them and still keep the OD of the handguard small enough and smooth enough to be comfortable. The 12" length will cover the flash hider and some of a suppressor. The 15" will cover most of a suppressor but may be too long when using a flash hider.

Would also add a SLR Rifle Works Sentry 7 adjustable gas block. This will allow you to adjust the gas between sonic, subsonic, or completely off for extra quiet loads. Sentry 7 is adjustable from front which is also an added advantage.

Hope this helps. I'm sure others will join in with more suggestions.

[JeffWard]

Thanks Buzzie. Seems a 10.5" barrel with 1.5" added due to the break will bring it out to 12" for the hand-guard. Most of the Noveske style comps seem to run 1.35"ish in OD, and the suppressors run up to 1.5" OD (like Surefire and Gem-Tec). If I drop down to a 8.75" or 9" barrel, then likely a 9" hand-guard with about 1.5" of comp sticking out...

Running a Seekins, Samson, or Troy handguard, they all seem to run about a 1.5" inside diameter. I've seen the Troy and Samson guards with Gem-Tecs inside them, so I assume that's a safe call. All seem to run a full top rail, so I can run a flip up front site in front of my primary optic.

The barrels I'm seeing run either 1:7 or 1:8. Any major stabilization issues with a 1:8?

Jeff

[ScottB70]

Both twists will stabilize the range of bullets suitable for blackout. 1x7 will work better for longer/heavier bullets. 1x8 would be fine if you're planning on mostly shooting lighter bullets. I have a 1x7 and can shoot groups under 1"@100 yards with 110 grain varmageddons. It's really your call on how you foresee the purpose of the gun.

I have a provectus rail on my gun. I'm running the 13" so that once I get my suppressor on my 8" barrel, only 2"-3" of suppressor should stick out.

http://www.hdrifles.co/warehouse/index. ... -rail.html

[randyrucker]

I little info from AAC on subsonic cycling:

Assembly, Barrel, AR, 9”, 1:8”, Gas System, SCARmor, 300 BLK. AAC part #100903

The barrel of the AAC BLACKOUT upper is made from 4150 CMV and specially processed to have up to 60% more life
than a chrome-lined barrel and without the potential loss in accuracy from uneven coating. This processing also
results in extreme corrosion resistance.

AAC has developed subsonic and supersonic ammo that both function reliably (with and without an AAC 762-SDN-6 sound suppressor)
without the need for an adjustable gas block.

They used to make their 9 inch with a 1:8 twist then changed to 1:7 twist to allow for heavier projectiles.

[JeffWard]

I little info from AAC on subsonic cycling:

Assembly, Barrel, AR, 9”, 1:8”, Gas System, SCARmor, 300 BLK. AAC part #100903

The barrel of the AAC BLACKOUT upper is made from 4150 CMV and specially processed to have up to 60% more life
than a chrome-lined barrel and without the potential loss in accuracy from uneven coating. This processing also
results in extreme corrosion resistance.

AAC has developed subsonic and supersonic ammo that both function reliably (with and without an AAC 762-SDN-6 sound suppressor)
without the need for an adjustable gas block.

They used to make their 9 inch with a 1:8 twist then changed to 1:7 twist to allow for heavier projectiles.

Any word on what the gas port diameter is on their 9" 1:7?

[randyrucker]

I little info from AAC on subsonic cycling:

Assembly, Barrel, AR, 9”, 1:8”, Gas System, SCARmor, 300 BLK. AAC part #100903

The barrel of the AAC BLACKOUT upper is made from 4150 CMV and specially processed to have up to 60% more life
than a chrome-lined barrel and without the potential loss in accuracy from uneven coating. This processing also
results in extreme corrosion resistance.

AAC has developed subsonic and supersonic ammo that both function reliably (with and without an AAC 762-SDN-6 sound suppressor)
without the need for an adjustable gas block.

They used to make their 9 inch with a 1:8 twist then changed to 1:7 twist to allow for heavier projectiles.

Any word on what the gas port diameter is on their 9" 1:7?

Specifications:
Material: 4150 Chrome Moly Vanadium Steel
Contour: Medium
Chamber: 300 AAC Blackout
Twist Rate: 1 in 7"
Chrome Lined: No
Finish: Black Nitride / Melonite
M4 Feed Ramps: Yes
Muzzle Thread Pattern: 5/8"-24
Gas Port Location: Pistol
Exterior Diameter at Gas Port: 0.750"
Extras: Low Profile Gas Block, Gas Tube
Additional Info: Suitable for use w/Subsonic or Supersonic loads, either Suppressed or Unsuppressed


Hope that helps.

[rebel]

I built just what are you are talking about, for the same purpose. Mega upper, Mega lower, KAK Mr. Blonde barrel 9",CMMG lower parts, semi-bolt, and a KAK buffer tube kit built for the Sig Tac brace. Probably a bit overgassed, no adjustable gas block. Have only shot subs, my reloads and some Hornady stuff and have never had a FT feed or fire. I can pm you a pic if you want, or can shuffle through the AR pic thread and find it. I have been pleased with the build and it's reliability.
edit-whoops The gun cycles unsuppressed or with the can slick as a whistle.

[jwb47]

I have an 8.5 inch cmmg barrel and have no issues cycling subs and super sonics with or with out the can . alot of people's woes with this cartridge are self inflicted , instead of looking at the obvious they start chopping off recoil springs and drilling holes in there barrel or taking weight out of their buffer .
then you see the horror stories with guys using all kinds of bullets and powder combos that I still havent figured out where they come from.
randy gave you some good info and I would add that aac would be very hard to beat for a complete upper use the buffer and spring recomended by aac . find a good load to get a base line for function I would suggest starting off with 1680 powder and a 208-220 grain projectile for sub-sonic use once you establish your gun will cycle then you can experiment with different loads . I run some gemtech sub-sonic factory ammo through my 8.5 inch gun with and with out the suppresor and it worked fine if you feel the need to try some factory ammo first this is what I would try . also dont let the 1/7-1/8 thing weigh to much on your desicion I have 3 blackouts , 2 gas guns and a bolt two have 1/7 twist and another has a 1/7.5 they all shoot supersonic very accurate.

[Randygmn]

I am also new to the 300blk. I did a lot of research before buying (and if you know me, you what what a lot means, lol. It means, A LOT). Ultimately, I went with a completely manufactured AAC 9"pistol (w/brace). They developed the round. I'm not knowledgeable enough to deal with cycling problems, so buying a finished product THAT WORKS OUT OF THE BOX, was priority number 1 for me. So far, I've run supers and subs, sometimes staggered in the same mag (to try to achieve a failure) and it keeps running. Sure, you can build a less expensive gun, and for some, that'll be the ticket, but for me, the extra money spent brings me to a platform that I'll be able to handle trouble free.