I have a lower that's gathering dust, and want the most powerful upper without needing proprietary mags etc.
Is there anything significantly more than .300 Blackout out there now?

Googling the topic produces too many rabbit holes for my taste. Thanks

350 Legend almost does what you want. iirc, you can mod a PMAG or settle for reduced capacity.

Not sure anything else uses the same bolt and mags? Sounds like some may be able to use same bolt but need different mags. Some could probably use the same mags but need different bolts.

ETA 350 may be a choice if the mags don’t bulge? I had read you could put 6.8 in a 5.56 mag and tried with a Pmag but by round like 8 or 9 it bulged enough that it wouldn’t seat.

I had thought (likely wrongly) that there was at least one .45ish loading that fit the bill, albeit with only ten or so rounds per 30rd 5.56mm mag.

Guess I figured since .300 AAC is nearly neckless that someone had taken it to its logical conclusion by now.

Keep the info coming though, I appreciate the input.

Seems to me, unless your goal is suppress, may as well stick to the 223/5.56. One can also vary bullet type a fair bit to adjust for the task at hand.

I would think .458 Socom would be the biggest one that would fit the mag. The bolt I have no idea but probably .300BO as you note.

quote:

Originally posted by apprentice:
I had thought (likely wrongly) that there was at least one .45ish loading that fit the bill, albeit with only ten or so rounds per 30rd 5.56mm mag.

Guess I figured since .300 AAC is nearly neckless that someone had taken it to its logical conclusion by now.

Keep the info coming though, I appreciate the input.

I have a 450 Bushmaster, and you can maybe get three loaded in a standard .223 magazine before issues. You can get dedicated magazines, but I use Tromix followers. Lack of taper means limited loading in curved magazines, although I read you might can do that with SOCOM and the right follower for 10 in a 30-rounder, and I think Tromix recommends Lancers with their followers?

You mentioned "standard AR bolt/magazines" in the title, so that's why I mentioned 350 Legend bc it's got same rim diameter as 556. Thought maybe for spares, you were wanting to stick with one bolt face to keep handy.

I guess the WSSM family is probably the most potent, if one was willing to go balls-deep into reloading and sourcing the parts. Back in the day, Olympia was pimping their 300 OSSM, which they claimed was like 300 WM.

quote:

I would think .458 Socom would be the biggest one that would fit the mag. The bolt I have no idea but probably .300BO as you note.

Funny you should mention this round. On San Diego Gun Owners Radio last week the guy was carefully saying that a .458 SOCOM round magazine would hold only 10 rounds; and he has many magazines for .458 SOCOM, which coincidentally looks very much like a 30-round 5.56 mag. I guess it would help (in this restrictive state) to have an upper that shoots .458 SOCOM.