From what I understand, some barrel/upper makers are doing small gas ports to cycle supersonic ammunition, and will not work if shooting subsonic
Is this true?
I would like one that will let me shoot supersonic unsuppressed, and subsonic ONLY suppressed. Is that how they are made?
Will the upper function properly with a suppressor? Does the suppressor increase pressures enough to have it cycle reliably?
I did search, but I failed my search roll. Any help would be appreciated.
I don’t think you’re reading right. One of the stated benefits of 300 BLK is that it will cycle with both supersonic and subsonic. Some barrel makers are tuning their barrels so that the subsonic will only run with a can attached (like Noveske) but that should be pretty clearly stated on their websites.
Shiz
Also you have to understand that subs use less powder to be subs- so a larger port is needed to get enough gas to operate the AR with the use of a can. With full power supers there is more then enough gas to operate an AR. So shooting subs without a can is ify just cause you don’t have alot of gas to begin with. I don’t have that problem myself as I’m not going to get into subs. 
CMMG is the manufacturer the OP is referring to. They do make dedicated supersonic barrels and dedicated subsonic barrels, depending on gas length.
The vast majority of other barrels will cycle 220gr subs unsuppressed through supersonics. As stated Noveske makes their ports slightly smaller, such that most need a can to cycle subsonics.
How do I know if I will get one that works suppressed, with subs, and supers unsuppressed?
I don’t care about shooting subs unsuppressed.
(from the different responses in this thread, you can see my confusion…LOL!)
It would help to have actual products, with links, to what is confusing you.
As to how you would know, asking the manufacturer strikes me as the best way to know.
If CMMG is doing something funky I don’t know about it, or why they would, or if I’d buy from a company mucking around with shit, and I can’t find anything on their site about it at first glance. A 300 BLK barrel should cycle subs and supers, suppressed and non-suppressed. The only deviation that I think is acceptable is the Noveske version where the only thing that may not cycle is the subs un-suppressed.
Just like with 5.56 guns the obvious way to get a quality product that runs is to buy from known performers in the market. In this case I don’t think I’d buy a 300 from anyone other than AAC, Noveske, or Daniel Defense (if theirs are even out yet). Anyone else IMO is a crap-shoot.
I can’t find it now but for most of 2011 CMMG had a little chart on their website comparing gas system length for their 16" 300BLK barrels, and indicating that the pistol gas was not recommended for supersonic suppressed and that carbine gas was marginal for subsonic unsuppressed. Again, this was gas system length, not port size. OP is that what you’re talking about?
CMMG appears to have pulled all reference to the difference in their barrels from their website, so they might have finally seen the writing on the wall and gotten in line with the true intent of a 300 BLK rifle. However, I was able to find this page, which has the chart showing what the OP, SOG, and I are talking about:
http://www.gunsholstersandgear.com/2011/12/24/cmmg-300-blk-rifles/
There are range reports from purchasers of early CMMG 300 BLK supporting what you see on these charts, as well.
Still, it would be best to ask CMMG directly if they made a barrel you were interested in. There are a lot of other great choices out there, and the list keeps getting longer.
DD rifles are now available. Short review of one here:
That chart helped me quite a bit Quiet.
My 16" CMMG carbine length 300Blk will not cycle subsonics unsuppressed, supersonics run fine. I have read about some that have no problems cycling subs, and others that won’t.
Just remember that is how one company decided to make them.
Pistol gas on the 16" 300 BLK can be made to work with all ammo.
Thank you all.
Todd K.
have you shot the CMMG suppressed?