[FrankenMauser]

I don't see the gas impulse being all that different with 200+ gr subs than I do with a 7.5" 5.56 barrel running standard ammo. Gas port size is a variable that can be toyed with to fine tune it, too.
It'll probably be loud without the can, though. And always messy.

Bullets being unstable? Nah. Barrel length doesn't matter there. If you can reach the velocity necessary to stabilize with any given twist rate, the bullets are stable. It's that simple.
The goal is rotational velocity. To achieve the desired rotational velocity, all you need to do is match twist rate to muzzle velocity. The higher the muzzle velocity, the slower the twist rate can be. The faster the twist rate, the lower muzzle velocity can be.
Barrel length matters not.

The 5" barrel doesn't matter much for propellant burn, either. My go-to subsonic load is estimated by QuickLoad to only burn about 91% of the powder by the time the bullet leaves the muzzle at 10.5". If I change barrel length to 5", propellant burn only drops to 82%. Yes, there's a difference. But it's not huge. .300 Blk subs are, essentially, handgun cartridges. And, handgun cartridges, in general, burn 70% of their powder in the first 3 inches of barrel. Nearly all handgun cartridges, including 'magnums' with slow powders, burn 70-80% in the first 4 inches.

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9" inches is not enough for full burn. For 100% burn with most subs, you need a barrel in excess of 24". That's one of the 'unfriendly' byproducts of running low pressures with subs. Low pressure results in lower burn efficiency, which means even more barrel length is needed for full propellant burn than with full pressure [generally supersonic] loads.

If I take my go-to load again, as an example: It uses 10.5 gr H110 and only makes about 20k psi. 99% propellant burn takes 27 inches of barrel (at which point, the bullet is actually slowing down from bore friction!). But if I increase the powder charge to 15.3 gr to achieve maximum SAAMI pressure, the load, of course, goes supersonic. The bigger change, however, is efficiency. At max pressure, that load will have 98% propellant burn at 5", and almost 100% burn at 6.5".
Big difference.
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PMags?
.300 Blk is one of the two-and-a-half cartridges that I actually use PMags for (the others being .223 Rem and 5.56x45mm). They don't play nicely with my fat cartridges like .475 Tremor, nor 6x45mm or .17-223, nor some round-nose style bullets in .300 Blk. But they generally work just fine with long-nosed spitzers, VLDs/ELDs, and bullets designed specifically for the cartridge.
Without .300 Blk coming into the mix, I would have given away all but the 20-rounders by now.