[SpringerTGO]

I think it boils down to what you are going to use your rifle for and what barrel length you are going with.
Without a suppressor my 9" 556 is basically a novelty. Way too loud and way too much muzzle blast to be practical for anything but annoying others around me. 300blk is way more practical. As others have stated, 300blk is way more versatile than 5.56 as well. I wouldn't want to be shot by either caliber, but 300blk is the superior defense round.
I have the same rifle as nikerret with the standard 5.56 barrel and an 11" 300 barrel. I've never even used the 5.56 barrel. But I also have other 5.56 rifles.
Again, like others have said, 5.56 has it all over 300 for ammo cost and ammo availability. If you are using it for plinking then 5.56 is the way to go. Since you can't get a suppressor they are both equally fun to shoot out of standard length unsuppressed rifles.

 

[Matteo1371]

As someone else said 6.8 SPC, 6.5 Grendel are options. Even 6mm ARC is an option whether you go short barrel or not, if 556 is considered too diminutive. My 10.5” Grendel and ARC AR is one of my favorites.

 

[tenifglockfin]

If I was starting new, it would be 300.

 

[Maxx702]

I’m currently searching for parts to put together a rifle. My biggest decision at the moment is caliber. Ideally I want a do it all caliber but will most likely end up with other calibers down the road. For now I just need a do all option.

I’m down to 5.56 and 300 blackout. Minus ammo cost there really isn’t much of a difference for me since I don’t live in a free state. I won’t ever be able to suppress anything, I’m limited to 10 rounds regardless, etc. the only pro each has that applies (from what I can see) is 5.56 being cheaper and 300 being a better hunting cartridge.

In general is there really going to be much difference if you can’t suppress the 300? If I could it would be a no brainer.

Since you do live in a communist state and the pistol brace rule remains unsettled, maybe you should consider 5.56 vs .308.

.308 is a versatile round and lots of fun in the AR platform. Probably overkill for short / medium distance target shooting and ammo is considerably more $$ that 5.56, but that round pretty much can do it all if you are shooting under 800 yards.

I have a .300 BLK SBR, great round for that application, but IMO not as much so in a long rifle.

 

[kodhd]

If you are not shooting suppressed I would not get into the Blackout. I would get the 556 and pick up a 6.5 Grendel upper for hunting if you were looking to hunt with the AR platform. The Grendel is a very good medium game hunting round. I have all three rounds, BO for suppressed shooting, Grendel for Hunting, 556 for plinking/stockpiling ammo.

 

[NWBoon]

I would start your build with a 5.56mm upper. I’ve been experimenting with heavier bullets when they go on sale like the Palmetto State Armory 75 and 77 grain AAC and they are pretty accurate out of a 18” barrel. For me, 5.56mm is a fun and economical round to shoot.

 

[DeltaFiveBravo]

Supersonic .300 BLK is essentially 7.62x39.

I have two 6.8 SPC uppers - one 20", the other a 16" lightweight. 6.8 is a great general purpose hunting cartridge.

I also have an upper in .25-45 Sharps, which is a 5.56 case necked up to .25 caliber. Loaded with 87gr Speer Hot Cor softpoint, it's accurate, effective, and legal for deer.

But...

If I were to do it all over again today, I'd probably choose 6mm ARC. I'd do an identical 5.56 upper setup to practice with and a 6mm ARC upper to hunt with.

 

[Matteo1371]

Supersonic .300 BLK is essentially 7.62x39.

I have two 6.8 SPC uppers - one 20", the other a 16" lightweight. 6.8 is a great general purpose hunting cartridge.

I also have an upper in .25-45 Sharps, which is a 5.56 case necked up to .25 caliber. Loaded with 87gr Speer Hot Cor softpoint, it's accurate, effective, and legal for deer.

But...

If I were to do it all over again today, I'd probably choose 6mm ARC. I'd do an identical 5.56 upper setup to practice with and a 6mm ARC upper to hunt with.

I'm in the same boat with Grendel. If I were to do it all over again I'd forgo Grendel for ARC. Which is what I am slowly doing now.

 

[doggiecareman]

I live in a similar state, I have a Flightlite, SCR, and 223 and also bought a 350 legend, upper from Bear Creek Arsenal. 350 legends is your do all cartridge.

 

[Mayhem like Me]

SBR’s will not be a thing due to the non free state I live in.

Well that stinks 300BO really shines in SBRs /Pistols so barring shorter barrels 5.56 is my suggestion ,with 6.5 G or 6.8 if you want a small frame gun with more oomph.

 

[HOTHEAD]

I’m in Ny as well and wasn’t aware we could do SBR’s. I’m in the finger lakes region. Good to know I can add an sbr to my permit.

It's not an SBR 😎
It's a pistol, with a pistol brace that happens to look exactly like a stock. I have 3 with the SBA4 braces and in NY you just add them to your permit and you can keep a loaded AR pistol in your car. I'm on lake Ontario North of Rochester, right on the lake.

 

[D20]

.300blk either isn't reliable, or needs special mags, buffer spring, etc.

Won't be building another one.

Short barreled AR's aren't anything I'm going to build anymore either. Prefer 18" rifle.

What do you consider short?
Sorry OP, I wont hijack the thread.

 

[D20]

I’m currently searching for parts to put together a rifle. My biggest decision at the moment is caliber. Ideally I want a do it all caliber but will most likely end up with other calibers down the road. For now I just need a do all option.

I’m down to 5.56 and 300 blackout. Minus ammo cost there really isn’t much of a difference for me since I don’t live in a free state. I won’t ever be able to suppress anything, I’m limited to 10 rounds regardless, etc. the only pro each has that applies (from what I can see) is 5.56 being cheaper and 300 being a better hunting cartridge.

In general is there really going to be much difference if you can’t suppress the 300? If I could it would be a no brainer.

Browse Bear Creak Arsenal to check out their offerings so that you k ow what's available and possible with an AR15 lower.
You could also consider the AR10 platform for bigger cartridges.

 

[Matteo1371]

What do you consider short?
Sorry OP, I wont hijack the thread.

Wouldn’t anything sub 16” be considered a short barrel?

 

[Zerodefect]

14.5 or less.

No more. Just don't have the purpose of a rifle.

 

[GucciGlockMane]

First one, then the other?

 

[Firearmjunky22]

What do you consider short?
Sorry OP, I wont hijack the thread.

Have at it! Doesn’t hurt my feelings

 

[Bosch]

I'm only considering 300 for suppressed home defense. 5.56 is so much more economical. I've got way too many AR's as it is, another suppressor host upper is just waiting in the wings.

 

[TacticalSpeed]

5.56 no question

 

[Glolt20-91]

AR-10 for heavy 6.5 Creedmoor

AR-15 for feral hogs on the ranch using factory ammunition, I have 55gr/62gr Gold Dots, Winchester 64gr Power Point, and Underwood #422 62gr TSX (Barnes) bullets...Black HIlls also loads TSX bullets, IIRC.

In addition, Speer also offers 75gr Gold Dots...223 Gold Dots are very popular with LEAs.

Law Enforcement - Federal Premium LE, Speer LE, BLACKHAWK!, Eagle - Load Comparison

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I've researched a number of calibers in the AR15 platform and have chosen to stay with .223/5.56, but I also handload.