What 300BLK barrel do you have? I'm asking because you definitely cannot shoot supersonic 115 grainers AND 205-220 grain subsonic through the same barrel. There is such a weight, length and speed difference between those two loads you need a barrel for each load. A barrel that shoots the supersonic standard 300BLK loads cannot stabilize the long slow subsonic, they will tumble and keyhole badly.
Thats why you see dedicated subsonic 300BLK carbines, the have very fast twists to stabilize those long bullets, but they can only shoot the subs.
I only browsed about this & I was of the impression they can do both.
This is the upper I ordered & received just now.
I couldn’t legally shoot with suppressors anyway, but I’ll do more research when I get off work.
Edit:
Edit2:
@Michael88 I cannot find anything about the supersonic vs subsonic barrel except
this ad by someone. It does make sense to me though, maybe I'm just not googling the right terms. Meanwhile, I'll try emailing Bushmaster & see if they can tell me.
I lubed & installed the thing just now & so much for the quick swap on the uppers. I was bragging to my friends about how I'll be able to swap from .223 to Blackout, faster than you can load the magazine? It turns out that the alignment for the rear takedown pin must be off by a tenth of millimeter or something(can't see any misalignment with naked eye). I have to tap it with a punch & hammer. It loosened up somewhat after the tenth time, but it maybe a while before I'll be able to swap it out effortlessly without tools.
Edit3:
Here's a really good video. Thanks Michael. 👍
Edit4: Wow, even more education in the comment section.
Jeffro Bodene - "First, if you think one revolution per inch twist rate is going to make a huge difference in whether a subsonic bullet stablizes or not...think again.
The first barrel was a PSA 8.5 inch barrel shooting subsonics...8.5 inch. Although PSA is good for holiday sales on rifle parts I highly doubt their barrels are what you consider high quality or match grade. "Moparmadman" purchased a Noveske barrel which are a high quality, match grade barrel. I have been working with the 300 BLK for years and have tried more combinatons and wasted more money than I care to admit. Although the 300 BLK was designed for a 9 inch barrel I wouldn't go shorter than a 10 inch barrel. I also recommend a pistol length gas system no matter what the barrel lenght to ensure good cycling. My personal 300 BLK hunting rifle is a stainless steel, 16 inch, match grade AND a
1:10 twist, pistol length gas system with an adjustable gas block. I have shot everything from 110 gr supersonic to 208 gr subsonics without any issues and NO key-holes in the paper. I built this rifle for deer and hogs and had no intention of running subs.
"Barndweller" stated "he got bullets that were .002 larger in diameter and his problems went away." Sounds like the barrel was not manufactured to specifications. I have sent my share of barrels back to the manufacturer because of poor quality. Also, "Berger Bullets" have a stability calculator on their website...you ought to check it out and enter various numbers; you will be surprised at what will and will not stablize. Also, do not forget a good trigger. A good barrel and a good trigger will make or break your rifle. In fact, there is no reason a 300BLK will not shoot sub-MOA accuracy all day long if it is set up properly with the right components.
Shoot straight and be safe."
Reply from poster - "First, thank you for commenting. I appreciate users sharing their experience, in fact that is why I made this video. I see you are talking from a hunting prospective, and I absolutely think that how you intend to use the 300 BLK is exactly what you need to know to make your decisions. Your statement of "I had no intention of running subs" says all that you need to know. Your rifle is set up how "you" needed it to be. That is one of the things that drew me to this round as it is very versatile.
I was hoping, in my case, that It would be able to shoot anything and though it reasonably well, including subsonic projectiles. Speaking in general terms, to improve your stability you need to spin the bullets faster. When limiting the discussion to a specific projectile there are only 2 things that you can do to improve your stability, increase the twist rate, or increase the velocity. When trying to keep your round subsonic you basically have a ceiling of 1050 fps, leaving twist rate your only option left. One inch of twist increases your rotational velocity a little more than 14.2%, not exactly nothing, but you would hope not a life charger either.
As "Bob Doughty" pointed out below a lot of manufacturers have stopped offering the 1:8 twist rate, most notable AAC. The 300 AAC Blackout was created by Advanced Armament Corporation in cooperation with Remington Defense. Yes originally it was the 300 Whisper etc. (I don't want to get into he whole history which anyone can look up) but it says a lot when they don't offer a 1:8 twist any longer.
I know that there are a lot of different things that come into play, bearing surface, bullet length, however twist rate is important. Berger does have a stability calculator and it is based on the "miller twist rule" that I mention in the video. But, real life isn't always demonstrated perfectly in a online calculator. Note that the constant 30 is Miller's twist rule roughly approximating the velocity as 2800 ft/sec. Its not that the calculator is bad, it is based on stability of projectiles of a specific construction (basically a football) in supersonic flight.
I also agree as well that probably the PSA barrel is not a "match" barrel, but I have had excellent luck with PSA before and I will gladly buy from them again. PSA does not make their own barrels, the buy them, and it may be possible to return mine if I like, but frankly it shoots super sonic rounds just fine.
Please don't think that I am trying to fight, I just think that a lot of peoples ideal set up for using this as a "fun round" is on a SBR suppressed as a subsonic. Doing the "proper" paperwork requires $400 to the ATF before you start buying parts, which all of us would gladly put into our rifles, but sometimes we skimp on the barrel (or some other component) cause that's what we have. I know that there are 1:8 twists that will do just fine but if you are limiting yourself to a budget barrel which some of us do, bet on a 1:7 before you bet on a 1:8 if you want to run subsonic velocity.
Good groups to you."
Actually looks quite good, shot really well with it.
Personally I like more compact handguns. Since I want .45ACP + higher capacity, search for my go-to handgun has proved challenging. 
...lol... To add to the list of expensive guns have you considered the FNH FNX-45 Tactical? This variant comes with raised iron sights and a threaded barrel, whereas the non-tactical variant does not come with equipped with any of these features.
