I've got an 11.5" AR, but I keep being drawn to an 8-9" PDW sized .300. Frankly, I don't want to get into another caliber (.300), but it seems like it has a lot of potential in a PDW package for HD purposes, or I could stick a can on it and it would be pretty close in size to my 11.5" upper. Obviously with a can, I'd expect the .300 to be much, much improved over the 11.5" 5.56, but if I were to run it without a can, how does the muzzle blast/flash compare? I've read that the .300 comes close-ish to burning its powder in a barrel that length and is less prone to the flamethrower-effect of 5.56 SBRs.

Also, I have no interest whatsoever in subsonics. If you bring up subsonic ammo, you will not pass go, and you will not collect $200.

Indoor range, dim lighting. Both have almost no flash with A2 hider on 9.5BO or 11.5 556 w/ minimalist flash hider

Muzzle blast is perceived more by the shooter the shorter the barrel gets...in general.  Configuration can help mitigate or increase the blast felt by the shooter.  A muzzle brake typically increases the felt blast, where as a flash hider or no device directs the blast forward.  The cartridge and the powder play a role as well.  Non-reloaders have no control in this respect, but consider pistol powders typically have faster burn rates because they have short barrels.  Load a rifle caliber intended for 16"+ barrels into an 8 or 9 inch barrel and yeah, you get blast and a fireball as the powder isn't consumed in the barrel.

300 Blackout is known for its powder efficiency and even the newer 350 Legend is right there with it (go on youtube and watch the 2-part series called "350 Legend by the inch."  They take a 25" barrel, fire 5 rounds, record velocities, cut an inch off the barrel, rinse and repeat.  They take it down to 7 inches).

If you reload, it's possible loads could be created based on barrel length to maximize powder efficiency, but I haven't seen (not that I've really looked for it) any load data that breaks it down like that.  It would be nice to have such data, but it would require a lot of work and the right equipment to generate it and I'm not one to experiment with powders not listed in reloading data just to see if it works.  I like my fingers, eyeballs, face, and hands the way they are.

There is no comparison. .300 supers have less blast out of my plain muzzled 5” than the 11.5” 5.56 with a flash hider.

As for flash, I can’t say. I don’t shoot indoors or at night.

There is no flash or detectable “blast” from my 8” 300 Blackout upper firing supers.  300 Blackout is very efficient in burning powder, so it’s all burnt before the bullet gets to that 8” point.

5.56 ammunition is typically loaded for longer barrels, so an 11.5” barrel won’t give the powder enough length to burn completely, thus there’s a LOT of flash, plus “blast” as the powder finishes burning past the muzzle.

I don’t know if you could develop a load for 5.56 that would completely burn the powder in 11.5” of barrel and still give decent performance.

But blast cans like Noveske’s Flaming Pig do a good job of redirecting the flash and blast so you don’t deafen yourself on the range.  Other folks there will be happier too.  

And then, we have companies making devices that intentionally make things louder ande more "blasty", a la the Snake Hound "Loudener" brake...oy vey...

Perceived blast can depends on muzzle device. Open prong flash hiders and flash cans are the least obnoxious to the shooter. At unsuppressed levels a few dB doesn’t sound like much, but the actual absolute pressure difference in pressure units can be significant.  A 2dB gain from 165 to 167 dB does more hearing damage than a 2dB gain from 140 to 142 dB.

And my Lil' Gun loads with 120s out of an 8" barrel with an A2 flash hider are invisible...

I keep my H110 for .30 Carbine, since Lil' Gun isn't as schitzy with any supers as H110 is.

5.56 will have a lot more blast.   An 8-9" .300 is more in the ballpark of a 16" 5.56

Flash in the .300 will depend on the powder and muzzle device.

8” 300aac is much more tame. If you have the choice to shoot that or an 11.5” 5.56 inside, choose the blackout every time. It’s much more comparable to pistol blast/flash.