300 Blackout - 30-223 Question


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frankmako
November 19, 2012, 11:39 PM
I got a Remington XP-100 that i used in the early 80's to shoot Metallic Silhouette. The XP is in 30-223. I got all of the reloading items for the round and tons of formed brass. I am building a AR rifle and I got to thinking about dropping a 300 Blackout barrel on the gun. So the big question,, is the 30-223 round the same round as the 300 Blackout?

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cfullgraf
November 19, 2012, 11:55 PM
I am not specifically knowledgable about the 30-223 round but I would suspect it is not the same as the 300 BLK.

The 300 BLK is based on the 221 Remington Fireball case which would be shorter than 223 Remington case.

So, I suspect the reloading equipment for the 30-223 case would not work for the 300 BLK.

Hope this helps.

Etkini
November 20, 2012, 12:28 AM
There are so many wildcats it's hard to keep up with them, but I'm going to go out on a limb and say no, it is not compatible. I do believe I've seen a couple 30-223s, and the case is longer than the .300BLK.

With that said, isn't the 30-223 also known as the 7.62x39mmUSA? You could build an AR-15 for the 30-223 if you've already got the components and brass. Buying a barrel blank from DPMS (or anyone really) is easy to do, you'd just need to find someone who could chamber it for you.

Another alternative (which I think you already suggested doing) would be to see if you can re-chamber your XP-100 to .300BLK. If I recall they use basic Remington 700 actions, and there's plenty of .300BLK barrels out there - you'd just need to find a short one. I personally wouldn't use one in a barrel shorter than 9", you run the risk of it not stabilizing.

jmorris
November 20, 2012, 01:31 AM
The XP-100 is based off the 40x not the 700. 30-223 is longer than the 30-221/300whisper/300fireball/300warrior/300blackout.

helotaxi
November 20, 2012, 09:38 AM
I personally wouldn't use one in a barrel shorter than 9", you run the risk of it not stabilizing.Only a problem if the barrel twist is too slow and you're shooting long subsonic loads. The really short barrels use a 1:7 rate of twist to account for this making it really a non-issue.

The .30-223 is not nearly the same thing as the .300 BLK, and it won't fit in an AR even remotely. The loaded cartridge would run around a 1/4" too long to fit in the magazine even with a light short bullet. You might be able to make a round fit using 110gn round nose for the .30 M1 Carbine, but what's the point?

1858
November 21, 2012, 03:35 PM
Only a problem if the barrel twist is too slow and you're shooting long subsonic loads. The really short barrels use a 1:7 rate of twist to account for this making it really a non-issue.

If you run the numbers through the Miller Stability Formula, a 1:9 twist is more than fast enough to stabilize a 220gr SMK at 1,000 fps. The issue is not with gyroscopic stability but rather with dynamic stability which is much harder to fix because the center of pressure is well forward of the center of mass of the bullet. The trend now is to move to 1:5 twist barrels for SBRs and 1:7 for 16" barrels but neither twist addresses the fundamental issue with the 'heavy for caliber' bullet that is needed to reliably cycle the action.

helotaxi
November 22, 2012, 11:22 AM
That makes sense except that the force acting on the center of pressure decreases as the velocity decreases.

Thinking about it, what makes more sense is that the drag on the base of the bullet (which is very high in a supersonic bullet) acts to help keep the bullet nose forward. Just a thought.

1858
November 22, 2012, 12:55 PM
That makes sense except that the force acting on the center of pressure decreases as the velocity decreases.


Drag force is obviously a function of bullet velocity, but if you look at the velocity of a 220gr SMK (16" barrel) it doesn't lose much at all, even at 400 yards.

Muzzle: 1015 fps
100 yards: 985 fps
200 yards: 958 fps
300 yards: 933 fps
400 yards: 910 fps

helotaxi
November 22, 2012, 01:09 PM
I know that it doesn't. Taking advantage of that was the original idea that triggered the .300 Whisper in the first place. The higher the BC the less it loses as well. The .510 Whisper is amazing in this regard.

My point was that if the Miller index indicates that it should be stable, the difference in the center of pressure and CG of the bullet isn't the real problem since the force acting on the center of pressure is lower with the lower velocity. But I guess that the CoP moves forward as the bullet slows, making the problem a bit worse. An easy solution is to not use the 220SMK and go with the 208Amax or one of the heavy Bergers instead.

1858
November 22, 2012, 01:21 PM
My point was that if the Miller index indicates that it should be stable, the difference in the center of pressure and CG of the bullet isn't the real problem since the force acting on the center of pressure is lower with the lower velocity. But I guess that the CoP moves forward as the bullet slows, making the problem a bit worse. An easy solution is to not use the 220SMK and go with the 208Amax or one of the heavy Bergers instead.

Actually, "the difference in the center of pressure and CG of the bullet" is the real problem. But it's not an easy fix for the Blackout when used in the AR platform due to the constraints of the magazine and action, assuming you want the rifle to work reliably, particularly in full auto. For the reloader who is looking to shoot the best groups possible, an A-MAX or Berger might help, but for "serious" use, sub moa subsonic accuracy simply isn't as important as reliability.

helotaxi
November 22, 2012, 10:33 PM
Actually, "the difference in the center of pressure and CG of the bullet" is the real problem. But it's not an easy fix for the Blackout when used in the AR platform due to the constraints of the magazine and action, assuming you want the rifle to work reliably, particularly in full auto.If that's the problem, then magazine length or any of that other stuff doesn't matter at all. The bullet would have the same issue from a .308 loaded subsonic. My question, then, is how am I just now hearing about this problem and why is it only a problem with the SBR's? How much velocity do the subs lose when you shave 6-7" off the barrel? It would seem that loads with 1680 for subs from a 16" barrel using 220SMKs don't fill the case and don't reach max pressure. You should be able to load the short barrel loads up some to get them just below mach and fix any stability issues that way. For the reloader who is looking to shoot the best groups possible, an A-MAX or Berger might help, but for "serious" use, sub moa subsonic accuracy simply isn't as important as reliability.I suggested those bullets because they are heavy enough to work as subs that will cycle an AR and are a different design from the 220SMK and thus will have different stability characteristics.

40 rod
November 24, 2012, 01:42 PM
http://snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=3067632

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