I have a can in ATF jail for .30 caliber. I've been looking at getting an upper in .300 whisper or blackout. However, *I* don't know shit about these newer calibers, which is better, yadda yadda yadda.

Using a standard AR platform, which is the hive's preferred caliber for cans? Pros/cons on both?

Thanks in advance!

300 Blackout is a fancy 7.62x39 imitation with sub-sonic options.  It's relatively popular, but ammo costs are still pretty high.  I've always found that the selling point of "you can use both super-sonic AND sub-sonic ammo without adjusting the gas!" to be a lie.  Everything changes about the gun when you switch from sub to super-sonic rounds.  Gas pressure, recoil, point of impact, reliability, blow back, range... everything.  I'd pick one or the other and stay with it.  Super-sonic or sub-sonic.  That being said, both suppress pretty good for 300 Black Out.  It does well out of shorter barrel lengths if you opt to go pistol or SBR.  Sub-sonic is obviously quieter, but turns the 300 BO into a pistol round.  Your .30 cal can should also work with .308 and 7.62x39.  Might give those a shot.  Much cheaper ammo.

Ok, good info, thanks.

How well will .308 or 7.62x39 suppress? I won't be shooting long distance with this can, so I'm hoping to avoid the dreaded sound crack of a rifle caliber.

My 300BLK experience is about the opposite of Kiran’s.

My 300BLKs have been 100%?reliable supersonic suppressed and unsuppressed, subsonic suppressed. They have been less than 100% subsonic unsuppressed, but an adjustable block doesn’t increase potential gas, only decreases it. And it a moot issue anyway since there is zero reason for shooting subsonic unsuppressed.

POI will change between supersonic and subsonic, but if you record your adjustment you can make it without re-zeroing.

I truly believe in adjustable gas blocks for shooting suppressed vs unsuppressed and all of my non 300 uppers except one have an adjustable block, but the 309BLK is the one round that does not benefit significantly from an adjustable block.

So... your "exact opposite" experience has been the exact same experience as mine.  O...k...

Read your own post.

Should I look at piston operated upper(s) instead of direct impingement?

Will bbl length affect reliability with a can? Can I look at pistol uppers? Gimme some skinny, fellas......  

No need for a piston and little benefit wrt reduced gas in the face.

My 300BLKs have been 9.5” and 10.5” SBRs. With suppressor those lengths make for a great relatively light and handy package about th length of a 14.5” or 16” carbine.

Good info, thanks.

Last stupid question. Is there a difference between .300 Blackout & .300 Whisper? If so, what?

Whisper was developed first, I believe it’s mostly moved to “300 blackout” now which was created by AAC; though I think whisper may still exist (someone correct me if I’m wrong)
There are very minuscule differences, I think some of the dimensions are slightly different in the casing. I personally would go with 300BO. And I have read before that 300 whisper can be shot through a 300BO setup (again someone chime in if I’m incorrect)

I hated 300 until I shot it. Then I started saving for a can so I could get the most out of it. 300BO subs are a touch better than 45ACP. same weight and speed bullet, better BC and penetration. Really easy on the ears without a can, fun at the range, in the woods, or if you're a secret squirrel with a can. With just a change of mags you can bump to supers, which are the ballistic equivalent of about 30/30 or 7.62x39. If your gun will run with unsupressed subs it should be fine with any supers and when you add a can. Maybe extra gas, but you can figure that out.

Where it excels is in making a short weapon. Shorty ARs take a round designed for 16-20" barrel and try to make it work in short. Becomes violently loud/percussive since powder burns outside the barrel. It sucks. Slow it down to subsonic and you basically have 22lr. my 300BO will be shorter than a 14.5" PW all said and done, but have more punch close range. I don't dump mag after mag anymore. I shoot rifles (besides 22) like 1x a year, so the extra 24cents a round it costs over 223 isn't going to financially cripple me. I'm happy with 9mm or 45 as carry guns, so I see no reason to worry that 300BO subs aren't enough. but I could always have the first 2 in the mag be subs and the rest supers if I really think I need to do some work in a hurry. I use my AR for HD and being shorter, lighter, quieter, but still able to be effective inside 100 yards is what I'm after.

This is my upper on a noveske folder pistol. It's shorter than an 11.5" 223 upper alone. Hard to argue.

Love the folder pistol!  

Thanks for the good info as well. I haven't yet decided on a host, whether to just buy an upper for a rifle lower or build a pistol. I may have to kick that one around some more, I think.

Thanks for all whom have added to my info base. I greatly appreciate it.