I'm embarking on my first 300 blackout pistol build.  What is a good sling for sling for additional bracing/stability while shooting?

There are dozens on the market and another one almost every month. The real decision is how will you sling up to use a pistol?

Not so fast, think about it.

Can't use the tube against your shoulder? Or, won't? Then one that pulls the rifle back toward you works directly with the recoil impulse and you then have to brace forward to counteract both making it even tighter in application.

Can you have the sling pull forward? That mitigates recoil, which is nice if you shoot nose to the charging handle, and it stabilizes, too. Your cheek rest handles the rear.

The issue is that without a stock we are trying to keep the rear sight from moving around - when recoil isn't a major issue, and with 5.56 it isn't, then what the sling needs to do - firmly control the motion of the weapon to help stabilize it - don't get locked into having to mount and use it like a gun with a stock. It doesn't have one.

Also goes to the issue of slinging up, which handgunners have yet to adopt, and they seem to do ok. Used as a higher powered high capacity pistol - it is - then you only need pistol accuracy, which is commonly 3-4" groups at 7 yards. Thats 21 feet, not 25 yards. Treat the pistol as a pistol with pistol engagement ranges and let the concept of it being a short barreled main battle rifle for what it is, a fantasy.

Nonsense. Most AR pistols are capable of carbine-like accuracy and a sling can help.

Most any single point sling that isn't too springy is fine.

Never tried using a sling to brace with, but using the cheek weld has worked great by itself.  I like that better than shouldering a standard length AR.

i use a sling , i bought it at walmart  around 7.00 for the sling it is a cheap hunting sling and the shoulder strap stretches , and the quick snaps around 2.79
each if i remember right , i can draw it in for a snug fit when i throw it over my back or adjust it so it as i push it forward it is a tight when i cheek weld the blade ..
dont have a pic but i put the front of the sling over the light and it's a
taunt fit where it lays flat along the side of the pistol so it doesnt just
hang down like when you want to carry inside a truck etc..
for me it does improve the groups when i shoot at 50 -100 yds .
i wouldn't want a AR pistol with out a sling but that's my choice .

TS2

I think it's safe to say that a 11.5" barreled 5.56 with rifle sights has a bit more reach and terminal effectiveness than your typical 9mm pistol that you carry concealed.

The term "pistol" for these weapons is a misnomer - it is simply a legal term to define the weapon, not a description of capability.  A street legal Ford GT has "passenger car" listed on its registration but no one will confuse its capabilities with those of a Honda Civic.

Rifle-caliber AR "pistols" are simply SBRs without a stock, all due to the way firearms are legally classified in the U.S. Properly configured and with good technique, they can effectively fill the PDW role.