I am looking to build the following 300 blackout… please let me know if you have any thoughts on the combo - primarily from a function standpoint. I am also curious about the barrel/handguard fitment as I would like them to be as flush as possible when attaching a barrel device or suppressor. I am also looking at a JK Armament solvent trap and would like any comments anyone has about those as well as how they may work with this build. Thank you!

Complete Lower: PSA AR-15 Complete MOE EPT Pistol Lower

Stripped Upper:  Aero Precision M4E1

Fore Grip: ION 7.5" HDX MLOK Handguard

Barrel: 8.3" .300 Blackout BA Hanson Pistol Length Performance AR 15 Barrel  w/ Superlative Arms Block and gas tube

Gas Block: Adjustable Gas Blocks - Direct Impingement - Bleed Off – Superlative Arms LLC.  Included with barrel

BCG: Bootleg Inc Four Position Adjustable Carrier

CH: Radian Raptor SD

You don't need an adjustable gas block AND an adjustable carrier.  One or the other if fine.

I was curious about that. Thank you.

Are you suppressing it?

Subs only? Supers only? Mix of both?

What buffer?

I’d say it will spend 60-70% of the time suppressed. I figured I’d just use the buffer that came with the SB-HDPDW but am open to other options. That lower is the only one I have ordered and in hind sight I wish I had ordered an Aero and then put my own SBA3 on it… still tempted to do that and keep this other as well.

Go with the adjustable gas block, but you don't need an adjustable carrier too.  If deciding between the two, the gas block will work and have the benefit of reducing the gas and crud getting into the receiver (if you even need to reduce gas at all).  The carrier is more convenient to adjust, but does nothing to reduce crud getting into the receiver.  Another potential downside is that the gas block, especially in pistol length, will erode and cause issues with the adjustments.  How long it'll last will depend a great deal on how much and how often you shoot it.  Some will argue that if you want this to be a "service" or "duty" weapon, then an AGB is an unacceptable failure point.  

If you aren't relying on it to save your life and it's more of a range toy kind of thing, then have at it, go bonkers and try different things (that's part of the fun).  Me personally, if building a 300 pistol, I would build and tune it for either super sonic or subsonic shooting/suppressed.  I would certainly try it in the opposite side and if it runs, great, if not, then I wouldn't worry about it or try to tweak on it to make it do both well.  But that's me, you do you.

First, you need to determine if you need to adjust gas at all. In suppressed shooting, you may not need to, or you could even be under gassed, but that depends on several variables that you won't know until you assemble and shoot it.  You might also have to play with springs and buffers as well.

A benefit of 300 BO is the wide range of bullets it can shoot, but that same versatility becomes a hinderence when you try to run it with suppressed heavy bullets all the way to light super sonics all in the same gun.  You're asking a lot of the system in that case.  Some builds can pull it off, some will constantly have issues in one regime or the other.   Like everything, it's a trade off, but not impossible to do.

I'd go adjustable carrier over AGB.

I have a longer rail on mine, and the AGB let out gas, which caused my rail to heat up insanely fast. Granted you can put an AGB on restrict mode, but the carrier still wins in the ease of adjustability dept.

My 300 BLK has a Bootleg carrier with an LMT Enhanced bolt.

This is the way