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Building a lower for an SBR what do I need to know?


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What's different from a standard carbine? Heavier buffer?


Not necessarily.  There's literally ONE part that's different than any standard AR-15  = the barrel.  I built Dawn Marie's 14.5" 300BLK rifle and started it with a standard carbine buffer.  It's a 14.5" carbine gas setup, I load the bullets with 150gr projos and a little hot. I felt I could soften it up a little for her, so I added an H2 buffer to it.  Shot great before, shoots a little better for her now, more controllable.


I'd say start with a standard buffer if you want, see how it does, and move to an H or H2 buffer later if you feel the need.  .

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Well, if it's a 14.5 it's easy to "legalize" by pinning on a muzzle device of the right length without going through the NFA hullabaloo. I suppose NTX is looking for an even shorter setup.


For the 300 BLK build, I'd recommend a pistol length gas system with as light a buffer as possible. Go light on the BCG too if you can. Going this route (reduced weight reciprocating components) makes the 300BLK rifle more reliable for low-powered subsonic rounds, and you should be able to switch between supersonic and subsonic rounds (both firing unsuppressed) without futzing with anything.


I love my 300 BLK setup. Shoots like clockwork and recoil is like a .22LR!!

Edited by shibiwan
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What I notice from having shot a SBR in Mass. is that the case ejects sharper to the rear almost 5:00 position.The upper was not slick side.I'll ask brother Ron to make sure later this morning.As a lefty myself all the other rifle's in mid-lenght and rifle-length gas eject more to the 4:00 position.I think the extream rearward ejection of case is the nature of the beast for SBR. <dontknow>

Edited by unforgiven
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RRA just had a run of lefty uppers. STAG still puts out theirs every once in awhile.

If you're building a dedicated SBR, you might as well start in the right (or in your case, left) direction.

If you can grab an upper receiver and BCG, you can sit on those parts while you wait on your stamp. Finding them parts isn't an easy task... just ask Edgecrusher.

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RRA lefty only works with the lefty lower.Stag has a lefty,then change barrel and cut down.American spirit might have a lefty.Talked to brother Ron and his SBR's had a shell deflector on them.Or get a righty and improvise a shell deflector.Good luck brother NTX.

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Hey Tex. Stag is showing lefty upper receivers and bolt carrier's as in stock. Just to let you know. Can't get on American Spirit's sight right now...it appears to be down. As Robo said, start gathering parts while you wait for the stamp. They may evaporate from time to time.


What kind of barrel length are you looking for? Mine is a 10.5" Noveske barrel with a pistol length gas system. It'll eat up any kind of round I've fed to it so far. From the 110's supersonic, to the 220's subsonic (suppressed and unsuppressed). Here's an example of the overall length of mine. Keep in mind that I swapped the stocks out from a MOE to an ACS in between the pic taken. The can (AAC 762-SD) only adds about 7 inches to the OAL. My AAC SDN-6 adds about an inch or so less.





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Nope. Barrel extension will be the same regardless. Just the carrier and upper need to be different. Balance on mine is just fine. She's pretty lightweight overall. And you don't have to take my word for it. I thought that Robo was gonna steal the thing from me once he started playing with it. Though I think he preferred the MK18. He was down right horny for that thing. Once he cleaned it that is. <lmao>

But that's another story.

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That Mk18 was pure sex for me. So dirty I damn near had to mortar it loose... only to make her extra dirty again.

Daniel Defense makes an awesome weapon. I'm especially impressed with their CHF barrels. The 10.3" seemed to be the sweet spot for me, although I think I could go to 11.5" without noticing much difference.

Edited by Robocop1051
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You know.... If you only want to buy and wait for one stamp... I think it was Spikes that sold a 9" 300Blk barrel with a suppressor welded to the end. The combo actually measures OAL as a 16.25" barrel, so you only need a suppressor stamp.

You won't be able to swap the suppressor between rifles, but you'd still have one badass machine on your hands.

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AAC-762-sdn-6 is the bad boy you want and a AAC 51T-Brakeout flash hider /adapter

I agree with the suppressor, but this is a better brake/adapter...


The brake acts as the first two baffles of the suppressor. This increases the lifespan of the can... it's also one hellofa muzzle device!

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