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Discussion Starter · #1 ·
Came across an article that measured actual results from carbine-length guns in 5.56 & .300BK, and also compared pistol-length guns in both calibers as well. The only inconsistent aspect was that in the pistol-length barrels, the 5.56 was a 10", while the .300BK was only a 9"; which gives a slight advantage to the 5.56 when doing comparisons. But even with that one minor discrepancy skewing things slightly in favor of the 5.56, the results were still completely predictable. They tested 55-grain commercial .223 fmj and full milspec 5.56 M855 62-grain ammo in the 223/5.56 guns, and used generic UMC 115-grain fmj in the .300BK.

Short version, in the 16" barrels, the .300bk exceeded both the 55-grain 223 and 62-grain 5.56, including running right at 60% more powerful than the 55-grain stuff. In the pistol-length barrels (again, even with the .300bk having a shorter barrel than the 5.56), power-wise it still beat the full milspec 5.56 M855 by more than 30% and beat the 55-grain 223 by almost 40%. If the barrel lengths were the same between calibers, the gap would have been even wider.

Fwiw - and spoiler alert - the summation of the article (emphasis added):
So, what were my conclusions? Well, first of all, a 115 grain 300 BLK round has more muzzle energy inch for inch than anything in the 5.56 or .223 range. No real surprise there.

...What’s more interesting is what happens when you cut the barrel down. The 300 BLK’s muzzle energy out of a nine inch barrel was within 100 fps of the 62 grain 5.56 coming out of a 16 inch barrel and when compared to the 62 grain 5.56 out of the 10 inch barrel, it was no contest. The 55 grain .223 bullet isn’t even in the game when compared to the performance of the 300 BLK.

...In an AR-15 platform, there really is nothing that the 5.56 round can do that the 300 BLK can’t do better. Sure, for very long distances, the 5.56 might have a slight advantage, but with his 9 inch barrel, Foghorn was able to ring the 500 yard gong with supersonic ammo and could consistently hit the 250 yard gong shooting the slower subsonic ammo. We already know from the calculations above that once you cut the barrel down on a 5.56 round, you really impact its performance, so to keep up with the 9 inch 300 BLK SBR, you would need to retain your 16 inch barrel for 5.56.
Interesting that I'd never heard it put that way - that to match an sbr-length .300bk, you need a 16" 5.56. Which is right at true; I'd just never heard it put in those terms before. Running 1125 ft/lbs, a 9" pistol or sbr in 300bk pretty much matches a full 16" gun in 5.56 caliber.

Again, for full disclosure - my primary gun is a 16" .223 and I have no intention of changing that. But when a more compact gun is called for, (truck gun, ghb gun, etc...?) the short .300BK is the bigger 223's equal in many ways, and is even its better in a few aspects.

The article: SBR Caliber Showdown: 5.56/.223 vs. 300 BLK - The Truth About Guns
 

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One reason why I'm slowly going to the .300BO. Maybe slower than I like or want but it is what it is.
 

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This is why I really like both of my .300 blackouts. The pistol has proven deadly against javelina, suppressed.

it’s also why I’m considering swapping out my .223 for .300 blk in my carrier.
 

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If your goal is a short barreled weapon with real performance, then the .300 is it. if you want a short barreled rifle with lousy performance, then the 10" AR in .223 is it. You really can't argue it any other way. Unless, of course, you believe in the necessity of neutering your defensive rifle with a .22 unit, that will get you killed.
 

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Discussion Starter · #5 ·
One reason why I'm slowly going to the .300BO. Maybe slower than I like or want but it is what it is.
I put off getting into .300BK for several years after shooting my first one. Mainly to avoid possible complications of having two identical platforms using two different calibers. But the numbers being what they are, I finally gave in and bought an inexpensive 16" upper (for $250 iirc) at a gun show probably 8-9 years ago. After that, I impulse-bought a single-shot break-open gun in the caliber, because it was crazy cheap and came factory threaded for a suppressor. It's an ugly little gun, but it's a great tractor or four-wheeler gun.

Main advantage (only advantage, really) that .223 has over .300BK for a private citizen's use is ammo cost. The lower cost per round, plus the fact that I've shot it for so much longer than I have blackout, means I have a lot deeper stash of it on hand. That's a valid consideration and a big advantage in favor of the .223, but imo it's about the only one. Even with the much-longer history in the market that the .223 has, the .300 still has substantially wider range of ammo options; if we're measuring on a performance basis. You can use a load with tons of penetration like a dangerous-game bullet, you can get a load that's good for varmints, and you can get loads that run pretty much anywhere in between. On a round-per-round basis, my 8.5" blackout pistol slightly surpasses the capability of my 16" .223 carbine, and a 16" .300BK carbine outdoes them both.

The 223's weight advantage - for a private citizen - is pretty much a moot point. A private citizen isn't going to be humping their life on their back or carrying a full combat load of ammunition. But even if carrying three loaded 30-round magazines, the weight difference between M193 and 115-grain .300BK ammo is 7.9 ounces. If eight ounces makes a difference, the problem isn't the caliber; the problem is weakness. If I were in the army and carrying as many mags as I would in that situation, sure, the weight could become an issue. But has any private citizen anywhere in the US ever needed (and so needed to carry) more than three 30-round AR15 magazines to resolve any crisis situation? I can't think of any. Even the Sutherland Springs church shooting - where the good guy and bad guy were both using AR's - the good guy never even used his entire first magazine.
 
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Well foot I had some stuff typed but it's .. gone.

Redo:
I got the barrel a few months back. I'm just waiting for prices to get a tad more reasonable on stuff.
A local buddy of mine has a lot of experience reworking cases from 5.56NATO to .300BO so at some point I'll get with him to take notes and make a list of what I need besides what I already have.
Re carrying ammo: I agree with 3 mags. It's been discussed on Warrior Talk that unless it's your job to kick ass and take names or all out civil war 3 is good enough.
Hell how many rds did young Kyle Rittenhouse fire off?
 

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I bought a small cut off saw and a jig on line for cutting .223 cases to make .300 blk cases.

once you get the saw and jig set up, you can cut a lot of cases very quickly.

I would cut a bunch, slightly long, then later run them through the sizing die.

then, at another sitting, I’d run them through the trimmer to get them to size.

then I’d check each one with a size checker.

after that, I’d use my normal loading sequence.

if you’re like me, you probably have buckets of .223 brass laying around.

I use H110 in my black out loads, with 125 gr bullets. It’s supersonic, but perfect for hunting.

I buy my subsonic loads.
 
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