I am thinking of building a 300blackout upper. It will be a 12-13.7" bbl w/ a carbine or mid length gas system underneath a t14" troy trx or vtac rail. My only road block is, I want to put a noveske kx3 on as my muzzle device to keep it non nfa. Since the 300blk and 556 can use the same bbl, would using this muzzle device be ok? Can you guys guve me any suggestions?
Can’t you SBR?
If you’re going with this round you’ll like it best with the 8.5" barrel. I put one together for a guy and it was pretty nice.
Yes I can but, I want a little bit more barrel just in case I want / have to take a shot past 300m for a more accurate shot. And… I don’t think I have the patience to wait for my stamp to get back. I’d end up buying something else to hold ke over 
Nobody ever says I wish I’d have built a longer gun… 
I do however read where people want to get the KX3 removed… and they want to get an SBR.
Just some food for thought.
13.7" w/Pig is a horrible configuration. You really should go shorter.
300blk an 5.56 do not use the same barrel
Barrel length wont affect 300blk accuracy
Middy gas is not an option in 300blk
You should really read up more on the cartridge before jumping into it. I love mine(11.3") by the way
I agree. I was being delicate… because I’m not very opinionated… but that’s what I wanted to say. 
We like the 8.2 or 10.2" with an AAC can personally. The round is awesome supressed, a lot of fun really.
Cool, I 'do some research before I start any process with this build. Thanks guys for the help!!
Cold, So which would you go with? Its looking like Washington State may get our SBR’s back, right now I have a 16" but want to build another, and I have had a heckuva time trying to decide which lentgh to use. Travis’s video with the 8" is pretty damn amzaing, so in a way maybe the 8" would be the ticket, but I just dont have any short barrel knowledge on this, I know my 16" runs great both suppressed, and non which is why I went this route anyway as 95%+ shooting is suppressed.
Mine is an 8.2 Noveske with an AAC 762n-6.
Very handy.
I’ve been running an 8" 1:8 twist Noveske (now sold as 8.2" 1:7 twist) for over a year, and it is a fantastic size. I actually had a KX3 on while waiting for my SDN-6 stamp, and it did a fine job of helping me keep friends at the range. Once I put the AAC brake on the end, I lost all ability to shoot that rifle canless with others present on the line. That has not been an issue since taking my SDN-6 home in May. 
I will have a fun video up by Monday doing some suppressed mag dumps of both subs and supers, testing out a Gen M3 PMAG. Even though I actually own and shoot this rifle, I was still giddy seeing how quiet it was on video. At $1/shot, I hadn’t done any full mag dumps of subs before, and it was awesome!
I also shot a box of 20 rounds at 100yds for a video on the repeatability of the LT104 QD scope mount, and I was easily getting under 1" with that 8" barrel and Remington 125gr match ammo. It matched my 16" 223 Wylde barrel from Rainier Arms shooting PMC 77gr match ammo, which means in the hands of a better shooter, both would be printing even smaller groups. I hope to have that vid up Sunday sometime.
After doing some reading, the blk out can be ran with a muzzle break and also surppressed. Well, I am not too concerned about the surpressed part but, having a muzzel break like the kx3 I mentioned works. As for barrel length, 9-11 would be fine for me . I guess I’ll be filing some paper work with the alphabet boys. Is there anyone out there who is running a kx3 on their blackout longer than 8"?