I have 5 AR lowers built as pistols.
1 9mm Brigade Arms with a 8" barrel that uses Glock Magazines, Romeo5, and a KAK Shockwave II brace.
1 5.56 with a PSA 16" upper with SS Barrel, Sig Whiskey 3 Scope, Fortis Muzzle Brake, and a SBA3 brace that may get swapped out to a carbine stock.
1 5.56 with a PSA 10.5" upper with a Romeo5, a dedicated suppressor, and a KAK Shockwave brace.
1 300BO with a PSA 7" upper with a Vortex Strike Eagle 1-6X24, an AAC 762 SDN-6 suppressor, and a SBA4 brace.
1 built lower with a SBA4 brace. I have an AAC blackout 7.62 51T, and a Vortex Strike Fire II that I am either going to put on a 10.5" or a 16" 300BO PSA upper.

I know that 16" is not necessary for 300BO and that the optimal barrel length is in the 7-8" range. Also all I have is 300 rounds of 220 Grain Subsonic Ammunition. If the ATF decides braces are all of a sudden bad I would most likely Form 1 the <16 inch pistols to SBR's.  That is part of my reasoning behind going 16" on this last one.

I could take the Strike Eagle off of the 7" and put it on the 16" and use the Strike Fire on the 7".

Thoughts and opinions.

If you want hedge your options if braces are ruled illegal, register one or two lowers as SBR.

16” Blackout uppers are good for hunting, when you want to maximize velocity.

Neither.

Unless your decision is driven solely by commonality of ammunition (and to an extent, magazines I suppose), I don't see the point of either in Blackout.

Short (8-9") in Blackout, anything over that length, there are just better options IMO, definitely dependent on what you want the thing to do. 300 Ham'r, 6 ARC, 6.5 Grendel, probably others I'm missing. That you can still find 6 ARC ammo is surprising to me, but another feather in its cap I suppose!

Blackout simply wastes a TON of barrel length for such minimal gains over 9" (to be fair, 10" is probably still getting you decent return on the extra length, but other cartridges start to come into the picture at that length) that it just doesn't hold much appeal *to me* in anything other than short barrels. Which you already own.

I can see the appeal if you want to keep to a minimum number of cartridges and you already have the Blackout, and need as much velocity as possible within that criteria. But in terms of trajectory, velocity, and energy at greater distances, the longer barrels are better served with different cartridges.

If all you have is subsonic ammo, go shorter. The only reason for a 16” 300blk is hunting with supers to maximize velocity.

This was my thoughts exactly. I was going to try to find Supersonic if I went with the 16" Upper, but I think that ship sailed a few months ago.

Yes, it is based on commonality of ammunition. I personally have no use for any other caliber than 5.56 or .300BO in an AR Platform.

Short it is! lol

I ended up building another 10.5" 5.56 upper with sort of an old school feel with Magpul Handguard to put on my built lower.

So 200 to 300 FPS with a 110 grain bullet going from a 9" to 16 Barrel" (+30 FPS per inch) 300 BLK is such a minimal gain.  Good to know your such a ballistics expert......  But a 5.56MM that loses 50 FPS + per inch going from 20" to 14.5" is just the bueno.....

Well if you believe that averaging fps gained by inch of barrel is appropriate and valid, and only do it for one barrel length, I can understand why you'd say that.

But that means we ignore the fact that the first 8.5" average to 258fps/inch. 30fps/inch kinda pales in comparison don't you think? If I average 2400fps over 16", I come up with 150fps/inch  See how one average alone doesn't tell the whole story?

Given that 6 ARC can push a 108gr bullet with far higher BC, at 2350FPS out of a 12" barrel, why would one argue 300BLK as an example of 16" barrel length being effective/efficient? 300BLK is efficient in short barrels when talking supersonic bullets. As barrel length increases, it's efficiency rapidly drops off and other available cartridges use that length more efficiently, providing greater velocity and energy.

You can attempt to make a point without being a jerk.

Do both lol! I started with a 16”, then built another 16” and now I’m doing a 10.5. I shoot supers only from my 16s but I’m throwing a 10.5 together for subs!