I know that 5.56 is pretty much eliminated.
For those of you with a 8" or 9" barrel SBR - Robinson Armament XCR-L Micro 7.5" barrel
what caliber did you go with?
how loud is it without a suppressor?
what muzzle device do you use?
what suppressor do you use?

The 6.8 is pretty much wasted on a barrel less than 11.5" so rule that one out, I think.

I have a 9" 300BLK. I haven't actually used it yet, but I can say it is a handy length. I would not shoot it unsuppressed since I have a suppressor for it.

JPK

I have an 8" 300BLK. I shoot it unsuppressed with 150gr .308 pulls and suppressed with 208gr A-max. Suppressor is a YHM QD Phantom M2. Very quiet. No hearing protection required at all, but I still wear it just in case. Never shot the subs for accuracy. Just for fun. Seems to be decent. For hunting, I use the 110 Barnes black tip. Still very quiet with supressor, but nothing close to the subs. Very accurate and quite deadly on hogs. (within 200yds).
I had the 9" on it, but decided to change rails and went 8". No difference at all in handling between the two. 300BLK is best between 8" and 9".

6.8 is probably better than you realize out of shorter barrels. It may be the best of the three mentioned out of a 7.5". You can get around 2600 fps with 90 grain TNT's out of a 7.5" if you reload. Everyone will have their own opinion tho.

6.8

I have a 10.5" 300 BLK and I shoot it with subs with and without the Saker 7.62 on it.

It is quiet without the Saker and almost silent with it even in an AR15.

If it was a bolt action (or pump) rifle it would be completely silent.

With the supers is a different story.

My Sig m400 Elite in 300blk (9" factory barrel) with a Sig 7.62 titanium direct-thread suppressor sounds like a pellet gun with subsonic rounds.

You can gear the rounds impacting a dirt berm at 100 yds and further.  Keep in mind Sig's 762Ti direct thread suppressor is a larger sized can, which helps  reduce sound, so I attribute the quietness to the suppressor, much more than the barrel length.

When I fire a supersonic through the same setup, it sounds like a rifle, but without the ear piercing "crack" like 556.

A huge issue with 300blk subsonic is they drop like a prom dress after normal shooting distances, and reducing the ideal 9" barrel length to 7.5" would likely make that drop off even worse, not to mention messing around with your optic to get both subs and supers dialed in on the same optic (there'd be even more difference between subs and supers).

Do you actually plan to shoot it frequently?  If so 5.56 is the obvious choice.  The ammo cost savings alone puts it head and shoulders above the other two options if this is a weapon you plan to train frequently with.

With good ammo a 7.5 5.56 is very viable proposition for defensive purposes out to 50+ yards.  Unless this is a work rifle or hunting rig you probably will not need more range than that.  And the ammo cost savings means you can train more with it and be more likely to make effective hits with it.

The biggest downside to 7.5 in 5.56 is the obnoxious blast but that can be mitigated very effectively with a linear comp or, better yet, a can.

I have a .223 Adams Arms 7.5" piston upper on one of my AR pistols. I run a Gemtech HALO on it and it's not so loud. 

6.8 develops 80% of it's potential velocity in the first 10" of barrel.




5.56 is fun and LOUD!  But if you're looking for terminal ballistics it becomes a subgun in that barrel length.

300BO subsonic is great in that length.
Supers don't get up and go very well.

I am not saying that .300 is the right round for OP's application, but if that last chart is using a 180gr supersonic projectile, then you're doing it wrong.

110gr is the way to go for supersonics, maybe 123-125gr


I run a really mild load with 18.5 grains of H110 and a 110 ballistic tipped projectile, and get (if I recall correctly) 2020 fps out of an 8 inch barrel.

Only posted that for representative purposes since it was easily found in an image search.
The one for the 6.8 isn't optimum either since its using the original 115 OTM round.

Howdy,

I don't own a short 5.56, but I do own 2 short 300blk uppers.  The first is a 8.5" with a YHM QD FH and the other is a 6.5' with an A2 style FH.

When I completed the 6.5" upper I was at the range with a fellow that had a 7.5" 5.56 upper and a Franklin Armory BFS.  That thing was loud.

I test fired the 6.5" 300 upper and it sounded about the same.  I then put the 8.5" upper on and it was much quieter, I couldn't believe the difference in sound between the two.  When I fired the subs it was of course quieter than the 5.56.  The suppers that I fired that day were the Rem 120gn green/white box and my 125gn handloads.  There was not much of a difference between these two loads as far as sound was concerned.

Hope this helps.


Semper Fi,

Luis R.

You planning on running it with a suppressor?  If so, how often?  What are you going to use it for?  I'm a big fan of the right tool for the job.  Up to you to define the job.

I don't like 556 in short barrels.  Painfully loud and hard on baffles.  I love my 12.5" 6.8, it is my go to with a TBAC Ultra 7 for hog hunting.  There aren't many cans rated for a 8" 6.8.  I;ve got an 8" and 9.5" 300BLK.  The 8" is an integral Liberty Leonids.  The 9.5" has a Liberty Mystic that pretty much lives on it (9mm can).  Subs are fun but close to useless except in very limited situations.  Hunting wise, it's akin to bow hunting, and doing it right means the right bullets, and those are expensive.  125 supers out of the shorter 300BLK set ups are good enough for hogs and deer out to 150 yards-ish.

i have a 5.56 sbr with 7.5 inch barrel and kak flash can. Its freaking loud as hell and the flash is huge too. BUT i like it. Its my current pdw until the mpx is finished.