OK. I already have a 5.56 SBR and want to set up for 300BLK.  I see that you can't use supersonic ammo with the pistol gas setup due to pressure spikes.  Can I just get a carbine length bbl/gas system and have ADCO chop it to 10.5 or would there be reliability issues?  I want to be able to run both at any time I want but have a short barrel all at the same time.



Someone point steer me right if I'm way off here.

Pistol length gas system with an adjustable gas block. Run anything that you want.

Not sure where you are getting the inability to use supers with pistol gas OP but please let them know the info below.  The info you are aware of was for the proprietary 300 Whisper from JD Jones.

The SAAMI-accepted 300 BLK is designed from the ground up to utilize pistol gas with subs and supers and be reliable in short barrels.  16" barrels need carbine gas but won't be reliable chopped shorter.

Adjustable gas is completely not necessary unless you get a custom one-off from a smith who isn't familiar with the port size requirements.  I would believe Noveske and AAC, who ship their barrels without an adjustable gas block, over random anonymous internet posters (me included).  Here is info from Noveske.


Noveske 8" Rifle

Please visit here for info straight from various reliable manufacturers as well as the cartridge designer/mastermind (RSilvers).

http://www.300blktalk.com/forum/

Getting my info from midway USA





http://www.midwayusa.com/product/466879/advanced-armament-co-aac-ar-15-pistol-barrel-assembly-300-aac-blackout-762x35mm-9-1-in-8-twist-matte





second paragraph. Also, CMMG does not recommend using supersonic ammo out of the pistol length gas systems.



Thank you for the info. I'll have to start reading more on it. I have everything I need minus a barrel and 762 suppressor.



I won't jump into this unless I know with absolute certainty that I can run full on supersonic ammo.
 

Interesting.  I hadn't seen that.  I'm going to post that over at 300 BLK Talk and will post the thread here for anyone to follow.  Hopefully someone from AAC can clear that up.

Thread that I posted.  RSilvers is good about chiming in on threads within a day or two if he has any info to share.

http://www.300blktalk.com/forum/viewtopic.php?f=128&t=78196&p=731235#p731235

I have a 10" Novekse, pistol lenth gas system, from the factory, that runs on subs with a can and supers with and without a can. flawlessly. Although the only compaint, my 3 prong flash hider has that nice tuning fork sound, but thats my fault because they all do that. I dont have an adjusable gas block.

You could use the CMMG pistol gas barrel for supers and subs if you cut it back to 10.5 inches.  At 16 inches it is over gassed with the pistol length port and 16 inches of barrel.  I am thinking though that cutting the 16 inch carbine gas barrel back to 10.5 would make it under gassed and would cause you problems.

Honestly, instead of trying to fool with the CMMG barrel, I would just talk to Aaron at Delta Company Arms and order the barrel you need for a lot less than the CMMG with an ADCO chop job.  $169 for a 10.3 inch barrel.  

http://www.deltacompanyarms.com/products/.300-BLK-barrels%2C-AR%252d15.html

how is the thread job on the deltacompanyarms bbls? I'm looking to suppress and don't want to deal with baffle strikes.



 

The reason CMMG doesn't recommend supers out of a pistol system with there barrels is that none of there barrels are under 16 inches.

If the barrel is 16 inches or more you would use a carbine gas port but going under 16" you would have to use a pistol gas port to get reliable cycling with both super and subs. Any competent 300 BLK manufacture knows this and would make sure to give you the right gas port for the length of barrel you would request from them.

I have shot hundreds of rounds through my Delta company arms 10" barrel with my Trident suppressor on the front and have had no problems with any hint of a baffle strike.

I cannot agree with that strongly enough.  I had a 10.3" Blackout built by Delta Company.  An adjustable gas block was recommended after discussing how I would use the upper with a wide variety of loads.  I was a bit concerned, but went with the adjustable block.  

Concerns:  1)  Hard to get it set up...wrong!  2) Adjustments would loosen just from shooting...wrong!  3) I would frequently have to adjust it...wrong!  4) It would carbon up and be hard to adjust...wrong!

Here's what I ended up with:  1 turn out from fully closed for supersonic hunting loads.  1 1/2 turns out from fully closed for subsonic semi-auto shooting.  3 turns out from fully closed for my plinking load with light GI frangible bullets.  Note:  4 turns out from fully closed is wide open for gas flow per the maker (PRI).  I've now fired it 1684 rounds.  

My next upper will be a 16" 300 Blackout...I will be ordering an adjustable gas block for that one also.  However, for most .223/5.56/6.8/7.62x39 shooting with full pressure ammo I feel an adjustable gas block is a not needed and an extra expense unless the upper comes standard with one like Kies uppers.

Definitely agree that an adjustable block can be useful for an advanced reloader using highly custom loads that fall outside the normal factory ammo parameters, such as 175 grain subs.  

For the average shooter such as myself, the added expense isn't a requirement if using factory ammo or reloads developed following vetted load data from major manufacturers that are proven to work in the AR gas system.

Clarification from RSilvers found at the link above:

I will ask them again to fix this. Yes, they copied and pasted CMMG text into the AAC description. It seems CMMG gassed their guns to shoot their light-weight subsonic ammo. AAC does not have this problem, and it has nothing to do with pistol-length gas in general, even for 16 inch barrels.

thanks for the reply and cross posting for me.  Kind of disheartening though. Even Noveskes web site says that it may not function with subs being unsuppressed. Weird.

No problem.  Glad that could get cleared up.  I have seen numerous reports of folks around the internet who have subs cycle fine in their barrels without suppressors in Noveske and AAC barrels.  The pressures are supposed to be right at the operating threshold of the AR's bolt system though and you wouldn't want to go into combat using strictly subs without the added reliability from the backpressure of a suppressor.

I'll check back sometime when I have shot my Noveske 8" barrel on my pistol build and it is broken in well.

sounds great. I guess that I'll just have to shoot suppressed all the time.

You can get them to run subs unsuppressed fine if you change to a lighter buffer/spring.  However, why would you want to shoot subsonic if you aren't using a suppressor?  Any decent 30 cal rifle suppressor can handle 300BLK subsonics all day long, so there isn't a real need to practice without one.  If you don't have a suppressor, subsonics don't really have much use.

If you really need to be able to shoot subs unsuppressed through supers suppressed without changing anything on your setup, go with an AAC or DCA barrel.

I have a 10.5" barrel built by TP55 from the quarterbore forum and it runs subsonic and supersonic with equal relaibility. I haven't shot any supersonic loads with a suppressor as SWR doesn't recommend it but it runs great suppressed with subs.



With pulled 147 gr FMJ's it's probably my favorite upper for my '16.

How do you like the trident on there using subs? I saw one with a booster for 500. Decent price?



 

I love my Trident and it sounds great on the 300 and with 147 and 158 gr 9mm loads. It's quiet enough I have shot it in my backyard while my friend's wife was about 25 feet away in the driveway and she had no idea what we were doing.



$500 with a LID is a pretty good deal, but If I were buying this can today I'd buy a Liberty Mystic or a SWR Octane. I think the ability to take is down for cleaning is important, especially considering how much fouling I've scraped off the blast baffle.

I too am shopping for parts to build a Blackout upper.

Like you, for some reason, I thought I needed to shoot unsuppressed subsonic rounds.
(I will be waiting for the stamp for my suppressor).
I figured the heavy bullets would be more accurate, since they won't have to jump to the lands.
I was even thinking of having the barrel gas port drilled out, then adding an adjustable gas block.

I finally realized that I can load heavy bullets with a smidge more powder,  making them slightly supersonic.
This way, I would be able to reliably cycle the weapon and use heavy bullets without a suppressor.