Every time I see someone ask about non-5.56 calibers on the AR-15 platform, there’s a legion of fanboys who start proclaiming that .300 AAC (aka, .300 BLK or Blackout or whatever) is the way to go. Let’s take a look at its capabilities:

• Subsonic .300AAC? All the ballistics and energy of .45ACP. Somewhat better penetration due to bullet profile, but I don’t get the hype at all. We’ve spent literally decades declaring PCCs and SMGs dead, and now this is the hotness?
• Supersonic .300AAC? Nearly indistinguishable from 7.62×39 in terms of ballistics, and if you’re OK with .308 bullets in a .311 bore, the bullet selection is the same.

Yes, .300AAC can be a .45 ACP and a 7.62×39 on demand. That’s the best cartridge design of the years 1911 and 1944, all in one gun. That’s not a compelling to me. But, OK… let’s say that is compelling to you. I can use my imagination!

Now, the follow-on argument… how are you going to leverage that sub-and-super capability? The fantasy I keep reading is “keep a mag of subs on your for when you have to go quiet”. It’s easy enough to switch the cartridges in your magazine, but how is your optic going to react? Supers and subs don’t have the same point of impact. You’d need to re-adjust, possibly re-zero your optic every time. You are not going to be switching between subs and supers on the fly, end of story. This completely eliminates the utility of .300 AAC to me; I may as well be switching uppers at that point.

With that out of the way, I’d like to talk about the non-5.56 calibers that I do think make sense in an AR-15:

• 9mm is a pretty good caliber for running a gun short and suppressed. It’s cheap, and the new MEAN EndoMag even allows you to run them with a CMMG Guard upper. There are also Glock magazine blocks, albeit with external mag releases (sort of like the old VM HyTech Uzi magwells I’ve previously discussed). While it’s tempting to get into power comparisons or whatever, in the end, pistol calibers are pistol calibers.
• I like 22lr, but I will admit that I’m using it less and less. It’s a nice caliber for getting new shooters in the door, or dumping some rounds down range, and it suppresses really well, but the recoil is so minimal that you’re not really developing skills. I don’t shoot small animals, but I guess it could be good for that.
• 6.5 Grendel is my new favorite. It has excellent range, plenty of energy, and doesn’t suffer from magazine capacity issues. Now that Wolf ammo is being imported in that caliber, it’s cheap enough that running it at classes is plausible – provided your range allows steel-jacketed ammo (I’d kill for non-magnetic!). You can also get Hornady match rounds at non-exorbitant pricing. IMHO, this affordable ammo makes it a much better pick than 6.8 SPC II.
• .458 SOCOM is my pick for running subsonic. It’s expensive stuff, unless you reload, but it’s one of the only subs for an AR-15 that actually hits like a rifle caliber. I feel like that’s a very unappreciated quality. Downside is the high cost and very limited magazine capacity – it would be interesting to see if someone could make someone sort of partially double-stack magazine for it, maybe by expanding it out past the magwell like the Surefire 5.56 quad stack mags. You’ve got supersonic options for it, too, but you’d run into the same issues as with .300 AAC vis a vis point of impact shifts. That said, it’s not an unreasonable choice if you want to maximize the size of a hole you’re putting in something. You can make a cogent argument that .50 Beowulf is a better choice if you’re going supersonic, though.

You are not a horrible person or a bad shooter if you’re using .300 AAC, 7.62×39, 5.45×39, 5.7×28, .45 ACP, etc. I just think that people sometimes are fooling themselves about the real utility of some of the more oddball calibers out there.