Edit: In looking at other responses maybe I misunderstood.
I don’t have my .300BLK yet, but I’m about to give you my vast amount of knowledge re: it. Actually, while I jest about my lack of experience with the cartridge I have studied it quite a bit. I’ve had plenty of time to do so while I wait for mine.
There are 2 schools of thought when it comes to hunting and dispatching animals, no matter the circumstance of their demise or the number of feet. One school says fast and light bullets and the other says slow, big, and heavy. To confuse the issue for .300BLK there is a 3rd school that uses slow heavy bullets that expand.
All 3 work, just differently. It’s been my experience that light and fast bullets work faster if the bullet is designed properly to expand for the velocity it’s being loaded for. With .300BLK they can easily reach 2k fps and that’s enough for actual stopping power. No handgun cartridges that most folks would consider for SD can be said to have actual stopping power. It’s extremely efficient to raise the velocity of the bullet to give it more energy, and more energy is a really good thing. Indoors a silenced round is good IMO. Yes, these can have the blast suppressed and the blast experienced indoors will destroy ears extremely fast. But the supersonic crack will still exist. But it’s much more ear friendly than without a can. I suppress my supersonic HD rifle to make it usable indoors.
In the slow, heavy category, they work, just slower since there is no temporary wound cavity to speak of and no stopping power as with the fast, light school. Too, penetration is sort of phenomenal since many, but not all, bullets used here won’t expand, they just zip right through. This would NOT be my first choice for home defense due to collateral damage. Remodeling rooms can get expensive and what isn’t lost to burglary will be spent on remodeling. Is a .30 cal effective in this subsonic category? I think I would want a bigger caliber, but that’s me. If I’m going to throw a rock I want it to be a big one to get the point across as fast as possible. Too, the only way to get more energy on target since this is subsonic limited is to make the projectile heavier. That gives even more penetration and I already discussed that potential problem. It’ll work though. Oh, and it can be nicely silenced making it nice on the ears.
In the 3rd category are heavy expanders. Much the same as above, except expansion cuts down on penetration. I haven’t tested these, as I already explained, but I like the idea. With a can on the gun they can be friendly to ears, reduce the remodeling greatly and should rearrange the innards of the target quite nicely. These bullets alone (for reloading) are quite expensive but available. Just do a 'net search and you’ll turn them up. I’m terrible at remembering names but is Lehigh one brand?
Too, and you didn’t ask this, there is a manufacturer making bullets specifically for .300BLK that are relatively inexpensive for practice and what not. What good is a cartridge that’s too expensive to shoot? Most heavy bullets for it are designed for much more critical roles and are priced that way. Using (and paying for) the same jacket required for a “reach out and touch” cartridge is huge overkill for .300BLK. Copper plated bullets are just the ticket though. Again, names and I don’t mix at all well… Let me see if I can find it. (Footsteps retreat, then come back minutes later.) Berries.
There are at least 2 retailers that carry them as of a year ago and probably more today.
Please let us know how you make out.
Edit: FWIW, I wouldn’t want to be hit with either a .300BLK or 9mm. I use that as the yardstick to judge effectiveness.