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Old March 20, 2014, 10:40 AM   #1
more_bullets
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300 blk sbr barrel length

I'm going to build an sbr in 300 blk but I'm not sure what barrel length I want...I was wondering what your favorite barrel lengths were and why! Thanks for the input! I'm thinking 10" probably.
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Old March 20, 2014, 11:27 AM   #2
balyon85
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Think the claim is all the powder is burnt in 9" of barrel, so I wouldn't go shorter than that.
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Old March 20, 2014, 11:48 AM   #3
Sharkbite
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I have (and LOVE) my 9" AAC upper

Ive done it into a pistol and now an SBR, suppressor on the way

As for ballistics, dont beleive the oft cited "powder is all burned in 8 inch" myth

Even if it all "burned" in 8 inches the gas volume continues to expand so you DO get velocity benefit from longer tubes.

That being said the gain is not as much as other calibers so the 300BO is the best of the SBR rounds.

If you are going with only 1 gun in this caliber and think you might want to put a suppressor on at some point, id go 9". Very nice short length rifle. Easy to manage. Balances well. Good ballistics outta this length. With the can it comes out to about the same length as a 16" gun

One thing... Make sure whatever barrel you get it is running pistol length gas system. Wont be any question on a 8-10" barrel but 16" come both pistol and carbine gassed. Carbine gas is MUCH more finicky about ammo and gas pressure.
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Old March 20, 2014, 01:06 PM   #4
more_bullets
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Thank you shark bite! Very helpful info! Sounds like 9" is the way to go. It will be suppressed at some point.
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Old March 20, 2014, 01:21 PM   #5
Theohazard
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I was planning to add some thoughts to this subject, but Sharkbite pretty much said everything I was going to say .

Though I want to strongly encourage you to make a suppressor a priority. The 300 Blackout is ideal with a can; it's stupid quiet with subs and with supers it's still a little quieter than a 5.56. I have an AAC SDN-6 that works great on both 300 Blackout and 5.56; you can put it on pretty much any caliber from .30-06 on down (though I highly recommend NOT putting it on a .22).
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Old March 20, 2014, 03:13 PM   #6
Sharkbite
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The stamp for my SDN-6 should be back this month (fingers crossed)

Theohazard and i are of a mind on this... Get that form 4 started. Ive got 400 rounds of subsonic 300 (208amax) just WAITING to go downrange. I cant even count the 125gn supers
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Old March 20, 2014, 08:05 PM   #7
balyon85
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Wish I had a stamp in to sbr my blackout over my PAP M92PV. Guess I need to get another form filed out soon.
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Old April 2, 2014, 06:08 AM   #8
razor488
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If you purchase an upper and lower to build a sbr, which part do you have to file a form 4 for?

I read it's the lower but the upper is the part with the short barrel?
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Old April 2, 2014, 12:28 PM   #9
Theohazard
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It's the lower that is registered as an SBR (and to build an SBR you'd use a Form 1; a Form 4 is for the transfer of an existing SBR). The barrel is just a part, whereas the lower is legally the firearm. And a short barrel isn't necessarily for an SBR, it's completely legal to put it on an AR pistol without any federal registration.

But here's where it can get tricky: If you have the parts required to make an SBR and there's no other explanation for you having them, then you could get in trouble for "constructive intent". Basically, they'd say you intended to make an unregistered SBR. So if you own a short barrel it's a good idea to also have a lower configured for pistol use (meaning the buffer tube can't accept a stock). That's the way most people do it while they're waiting for their tax stamp.
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Old April 2, 2014, 01:06 PM   #10
razor488
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Can I buy a new lower using a form 4? Or is form 1 the only way to go? You have to get it engraved right?
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Old April 2, 2014, 01:45 PM   #11
Sharkbite
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You could buy a registered SBR lower from a NFA dealer on a form 4. The same as buying a suppressor, but i dont know why you would with an AR platform

Just take a standard rifle lower and do an Eform 1. Easy pesy. Same wait time as a form 4 anyway

If you do a form 1 the lower needs to be engraved with your info (personal or trust) as you are making a new NFA weapon. If its an already made SBR lower it will already be engraved with whoever made it a SBR
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