The TL;DR is if you are thinking about buying/building a 300 BLK just do it. Even if you have never thought about it, you should still do it. They are wicked fun and useful to boot. 8-10" barrel, pistol gas, H3 buffer, go hog wild.
A few years back I missed out on a buck on the last day of deer season when he bolted across the road into a small clearing and I got hung up on the door bailing out of the truck with my standard 24" barrel bolt action hunting rifle. From that point on I knew I needed something more compact and maneuverable, even if its only function was as a truck gun, but I didn't yet know what the answer was. That following spring in a carbine class I got a chance to shoot another students suppressed 300 BLK pistol for a couple drills and I couldn't wipe the grin off my face for days. I knew then what the answer was to my truck gun question! I've now killed 3 deer with it (including one my uncle failed to kill with his 7mm mag... remember, shot placement kids). It lives between the shotgun seat and center console of the truck all hunting season and never gets in the way but is always ready to rock. As an added bonus I have (IMO) the best HD weapon at my disposal the rest of the year.
Below are my major build components and notes based on my use case and experience.
Lower: Aero Precision, generic LPK - zero issues
Trigger: Originally went with ALG-ACT - a little heavy for hunting/shot placement over 50ish yards IMO.
Replaced with Rise RA-14, DO NOT BUY THIS TRIGGER, it shoots great for the first 50ish rounds but as soon as any dirt or grit builds up (especially bad shooting suppressed) it fails to reset.
Replaced the Rise with a Larue MBT-2S on their black friday sale this year. Have only shot the Larue at the range so far but all indications are positive.
Grip: Magpul K - for a compact LOP on a pistol or SBR a grip as close to vertical as possible with no beavertail is key, especially when shooting from a squared up stance. Even my normal BCM mod 0 wasn't great. A2s or Magpul MOEs are no-go. Granted I could always increase the LOP but the entire point of this build is to be as compact/maneuverable as possible.
Buffer tube/brace: KAK Shockwave - no need to go fancy on this. And if one were to inadvertently shoulder the gun the blade happens to nest against a backpack shoulder strap or plate carrier strap very nicely. I purposefully kept my gun as a pistol so that it can legally be kept loaded in the truck. An employee at an LGS who offered a lot of valuable input for this build SBR'ed his and was kicking himself for it afterwards. Note, laws/regulations on this may vary by state.
Upper: Aero Precision w/ forward assist - in the future I'd probably go slick side in the interest of minimizing potential protrusions that can get hung up when bailing from the truck in a hurry.
BCG: Bogarted from an old 5.56 upper, no need to go fancy or special here, just get known good/reliable one.
Gas system: Pistol gas/BCM fixed gas block/H3 buffer weight - cycles everything, subs and supers, suppressed and unsuppressed. Only note when shooting subs unsuppressed, last round bolt hold open started to fail around 400 rounds mixed suppressed and unsuppressed without cleaning. A few shots of CLP and it was back up and running 100%. Technically it's probably slightly overgassed, but no worse than any 14.5 or 16" 5.56 carbine I've shot. Every time I see an adjustable gas block on sale I consider getting one to try and fine tune the gas, and then I remember that everything functions as is and decide the juice isn't worth the squeeze.
Barrel: BCM enhanced fluted 9" - eats everything I feed it, but seems to group better with 110gn supers/220gn subs (1-1.5 MOA) as opposed to 125/240gn (2-3 MOA)
Handguard: BCM KMR 8" - fit up with 9" barrel and suppressor is tits. If you're not going suppressed and use an extended arm/c-clamp grip, definitely get a handstop. Surefire X300 (when on truck/home duty, not hunting) in the 12 o'clock position also makes a good support hand index.
In retrospect just buying a BCM complete upper group would have been easier but they were around $750 at the time and hard to find in stock. I got a screaming deal on a blem Aero upper so was able to piece my upper together for under $550.
Suppressor/Muzzle device: SiCo Omega - At the time this was the best option for weight/length/acoustic performance and handling magnum calibers (it does double duty on my 300 WSM elk rifle). Still hard to go wrong with this can, but after some issues with the ASR, I decided to upgrade to the Dead Air Key-Mo attachment and brakes which are awesome. If I were to start from scratch I would just go straight to a Dead Air Sandman.
Optics: Holosun HS503G w/ Aimpoint 3x on flip to side. Obviously not a hard use optic but has handled 2.5 hunting seasons so far with no issues. I have a pretty bad astigmatism and red dots bloom or starburst really badly for me, but I can pick up the sharp edges of the chevron in the ACSS reticle no problem (the dots still starburst though).
I have had a few issues confirming antlers on deer in low light/in brushy terrain with only the 3x so midway through last season I switched to a Leupold Mark AR 1-4 Firedot and will likely continue with that next season, or possibly even upgrade to a 1-6. Outside of hunting season it still wears the red dot.