Alright I have a complete BCM 5.56 16" upper with a YHM 13" FFT.

What I want to do is replace the barrel and gas system for 300 Blackout subs. My understanding is everything else can remain.
I plan to only shoot SUBS solely to plink with my IR laser and PVS-14.

I am looking for a very affordable barrel and gas system. This will be only for night shooting.

Any suggestions? Gas system length?
Links to purchase would help out.

Thanks

I'm assuming unsuppressed?

For sub-sonics to even remotely fire reliably from any length .300 barrel I would recommend a pistol length gas system.
Supers can be made to work with a 16"barrel with a carbine length  gas system.

Get a pistol gas system and go with an adjustable gas block so that if you ever decide to run supers you can just turn the gas down.  Also remember that the longer the barrel, the more velocity you will get so I would suggest somewhere around 10".  Plan on a chrono to test your sub loads.  You will not be able to tell by ear if a round is going super or not.

I own 3 300 blacks, 2 with pistol and 1 with carbine...

All I can say is, the carbine is going bye bye and I am replacing with a pistol length gas system, which is what the caliber was designed for!

No I'm running suppressed.

It appears pistol length gas system is the way to go. I probably will not get an adjustable gas block because a good one is fairly expensive.

Cheap ones work fine, one of mine has the cheapest I could find and I adjusted it and put loctite on it and it has been going for over a year now.

I think I got that one at Joe Bob's for less than $20 bucks.

I'll check it out.

For a 16" barrel...1:7 or 1:8 twist?
Shooting only subs

I am running 1:8 barrels on all of mine.

I would go with a 1x7 twist with subs and pistol gas.

Check out barrels from KAK Industry's barrels.  I have the 16" and it's a really nice barrel.

I strongly recommend going with a pistol gas system.  As someone has said, "carbine gas can be made to work," but it's not a given, while pistol gas WILL work with a standard buffer and standard loads.  My 16" upper, running a standard, non-adjustable gas block, eats up UMC (Barnes-made for Remington) 120gr OTFBs as well as Remington's own (lesser quality but still functional) 220gr subs with no changes at all to any part of the rifle.

I haven't tried carbine gas with this caliber for a reason: there are so many threads here about "why won't my gun cycle with X", when those guns feature carbine gas systems.  The only pistol gas rifles I've read about not working have been in guns that someone decided "needed" a ton of fancy additions or expensive parts.  With standard parts and pistol gas, it should "just work."