Today I ordered a Mag Tactical Magnesium lower with the intent of filing a Form 1 to SBR it.  
This will be a .300blk dedicated suppressed platform.
The research I've done on .300blk all says that the standard round will have a full powder burn in 9 inches of barrel travel - so I want to stay at 9 to 10.5 inch barrel.
I'm not interested in purchasing a full and complete upper, as I always pick and choose my individual parts.

That being said, I want to ensure that I have a .300blk platform which will cycle the subsonic rounds when suppressed (and not have issues with non-subsonic when not suppressed.)

What length and gas block should I be considering when ordering parts to ensure the cycling of subsonic rounds?

Have you been here?

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

I have yet to assemble my first 300blk, I just got started in ARs.  Less than 500rds through the first one I've owned.  300blktalk seems to be the best forums for such details...

I have a pile of parts for the 300blk. First one is a 16" carbine.  Second one is in parts buying stage, will be a 10.5" pistol.

Depending on the gas port size in the barrel it should run subs and supers with standard carbine buffer and spring.

There are definitely cheaper setups i'm sure will work, but I have a 10.5" barrel with a .750 gas port.  I have a JP Silent spring.  

I bought a Syrac Adjustable just to make sure i had no issues since i had read a bunch of different things.  I set the gas block one time and it has been flawless for supers and suppressed subs without adjusting.

Pistol gas and a 9-10.5" barrel.  Standard spring and carbine buffer and you should be ok.

The 10.5" will have a slight edge all other things being equal, so if you dont care about absolute minimum overall length go 10.5" if you want to cycle subsonic w/o a can.

If you're going supressed its hard to get an 8" not to cycle.

Some great information so far - and now that I've had my evening coffee I have another question:

While I've been searching for barrels and such, I found the Noveske - and they have a rather dire warning:



Is this just standard corporate liability speak or are .300blk systems this damn picky?  I want something that works - not a finicky 2 year old with a temper....

Will I seriously be limited to two different kinds of suppressors?  Will a build that I make using another barrel if it isnt' Noveske be prone to such issues as baffle strikes on certain suppressors and not on others?

I have built a 9.5 and a 8.5 inch barreled AR and had no issue shooting subs and supers suppressed and unsupressed, using not as known barrels.

I have used only the standard carbine buffer and spring.  AAC recommends the H2 buffer for short barrels.  But I have not found it needed.

just go to Ranier arms and buy their complete 9" upper..mine worked perfectly

http://www.rainierarms.com/

Get a 1 - 7 twist barrel if you can.

I use the noveske 10.2 barrel with JP silent capture spring with no issues suppressed or unsuppressed. I also don't have any adjustable gas since it is not needed with 300blk. Suppressor is an octane 9hd until my GMT300 is approved.

Well, I ordered the Noveske 10.5 1:7 .300blk barrel.  

Going to look at that JP Silent capture spring that was mentioned above.  


Now another question comes to mind - the buffer tube - what length/size am I going to be limited to with the 10.5in barrel?

If it's not a standard carbine buffer kit I need to buy a different set of parts...

Just the standard carbine buffer tube.  If you go JP silent capture spring you will probably just need to put a couple quarters at the bottom of the tube take up a little play in it.

LOL - that reminds me of how we used to increase the cyclic rate on our M-60's "back in the day" by putting a quarter on either side of the gas puck to decrease the travel distance....  :)

I don't think I'll opt for the JP silent capture spring - at least not yet.  I'll go with a standard H or H2 buffer and spring for now and run that for a while to see if I think that I need to decrease the noise it makes.

With a mil spec buffer tube you won't need anything other than the jp silent capture spring. No quarters or spacers are needed. My buffer tube is a spikes mil spec buffer tube.

Jp silent capture spring not only gets rid of the spron noise but it also seems to make the gun have a smoother cycling.

I assume you mean a milspec carbine length buffer tube, correct?  Or are you using a standard length tube?

Get a barrel with a pistol-length gas port.  That's THE biggest issue most people have with cycling subsonic rounds with a 300 Blackout.  With the case volume and typical powders used in subs for 300 Blk, you sort of need the earlier gas impulse at higher pressures than you can get fro a carbine gas port.

A second issue might be the balance of buffer weight and spring strength against the gas system.  You'll see lots of people talking about "your gas port needs to be enlarged" and similar stuff, but the FIRST thing to do is to see what buffer you're using.  My 8" 300 Blackout upper runs supersonic rounds fine with a standard carbine buffer, but for some reason there are people who figure that a 3.5oz H buffer is too light for X and they go straight for a 5.4oz H3 buffer and add an extra power spring to boot.  I think that's just dumb.  Why go modifying anything when you don't know whether you need to modify anything?  

From the stuff I've read, it looks like the best approach is to get the gun running with one specific supersonic load, then start experimenting with different loads.  Be prepared to use one buffer for supers, and another for subs, and maybe even different buffers for various different loads.  But swapping out buffers is quick, easy, and reversible, while reaming a gas port is not!

Carbine length.

Same buffer works for sups and subs on my 300 blk.

Glad to hear it, but there's no guarantee that every gun will run every load with the same buffer.  When Blackout is a "more mature" caliber, we may see more consistency and need to do less fiddling.  So I point out that some tweaking, like possibly needing to swap out buffers, could be needed...

.750 is not the size of your gas port. It is the diameter of the barrel where gas block is located.

Fuck that noise. Arf is king.

pistol gas tube and a normal gas block.
rock and roll.
What suppressor?