What to know about .300 BO SBRs?

[GregSmith]

I'm dreaming of a .300 BO SBR with a nice can on it. I don't know that much about piecing them together so I thought I'd ask all of you.

Is .300 BO built off a .223 platform or a .300, or .308? (yeah, told you I didn't know much.) What should I know about ammo when it comes to home defense Vs hunting? Buffer, BCG considerations? Who makes a good barrel and how much does that matter on an 8"-10.5" barrel? Speaking of, is there an optimal barrel length and twist rate? What else am I not thinking of?

 

Thanks!

Edited September 28, 2016 by GregSmith

[NateTSU]

The 300 black out uses ar15 recievers and bolt. Do you reload? I recently  shot a 300 bo pistol that had a 9" barrel and a can and really liked it. Subs with the can were unbelievably quiet. If it were me and I was going to use the rifle in multiple applications I would go with a 300 black out.

[GregSmith]

No, I don't reload. I've shot a suppressed .300 BO a few times. I would love to have one. What buffer you using? Twist on the barrel?

[bmiller]

I went a little overboard on the .300 blk when it came out.  If you don't reload, I would suggest trying a 9mm sbr.  I completely understand the fascination, but there are several .300 blk that are laying around here gathering dust since the 9mm ar showed up!  .Ammo is everywhere, and much cheaper.  Hell, on second thought, buy both.?

[RangerTrace]

I'm waiting on stamps for my receiver and can, but I already bought a complete 8" upper from Aero Precision to stay within my budget.  I haven't shot it yet, but the upper looks as good as any other AR I've seen.  When mine is finished, it will be a Lone Star Armory lower, Aero upper, LAW Tactical folding stock and Dead Air Sandman S suppressor.  

I went with 300 BLK on this one because I wanted the super shorty to use as a backpack gun.  No doubt if you're just plinking, the 9mm AR (which I also have) is a cheaper option, but if you plan to use it for defense or hunting, I think the 300 is a far better option.  220 grain subs at 1000 FPS are much better than 147 grain 9mm subs at 950 FPS.  And most importantly to me, you can run lots of lighter bullets for super sonic applications.  

[Bryan 45]

I like shooting 110-125g fast projectiles out of my 300blk as much as I do subs.  They hit quite a bit harder than 223 at 100 yards.  300blk is so much more versatile than anything else in the small platform AR.

I have a 8.5" pistol.  It's scary quiet with subs, and shoots really well with supers.  

 

Edited September 28, 2016 by Bryan 45

[Tom Freeman]

7-8 twist barrel with pistol length gas system.

 

Be sure it is threaded 5/8-24 which is the norm for 308 cans.

 

Learn to reload.

[mostholycerebus]

The ONLY difference between 300BLK and 5.56 is the ammo, and the barrel.  Everything else is compatible.

The ideal barrel length is 16", just like the ideal barrel length for 5.56 is about 20".  Thats the best length to maximize the cartridge without getting ridiculous length.  For efficiency, the 300BLK in 10.5" is roughly equivalent to a 16" 5.56 barrel.  Users tend to like those two lengths, with 7.7"-9" being popular.  You can even go down to 6.5" or 5.5", though then you are dropping to somewhat-better-than pistol caliber carbine levels.    

[DWFAN]

8.5 inch barrel, custom shorter than standard Pistol Gas. 110gr bullets go almost 2300fps and i havent found a sub that wont cycle. I run 180gr Sierras for cheap plinking.



The cost of factory ammo doesnt seem to be coming down much, so +1 on reloading.

[prestonj12]

I have one of each. 8" SBR blackout with a can and a 9mm SBR 5.5" set up for a can that's still waiting on my stamp. The blackout runs better. The 9 is magazine finicky. The blackout is silly quiet with 220 grain leather head bullets over 10.4 of 1680.


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[NateTSU]

11 hours ago, GregSmith said:

No, I don't reload. I've shot a suppressed .300 BO a few times. I would love to have one. What buffer you using? Twist on the barrel?

My friend is using a factory carbine buffer with a adjustable gas block and a 8 or 9" barrel I don't remember exactly. I think there are a few company's making remans for the 300 now.

Has anyone thought of building a 7.62x39 SBR? Ammo would be super cheap and ballisticly similar to the 300. That could be a good alternative to non reloaders. 

[Paul-the new guy]

I can tell you first hand the more research you do the more frustrated you will be.  At least that was/is the case with me... I have built quite a few AR's in .223 so when I decided to build a BO I did it the same way. I used a 10.5" Black Hole barrel with pistol gas.  Pistol gas is the only way to go IMO. I used all the same stuff I would have used in a .223 build, JP low mass BCG, JP SC spring, SLR adjustable Ti gas block etc... I shot maybe 20-30 rounds, all un-suppressed and it ran fine.  I think the issues may come when I FINALLY get my can.  I have a full mass bolt and I have a JP H2 SC spring ready and waiting.  

[RangerTrace]

1 hour ago, NateTSU said:

My friend is using a factory carbine buffer with a adjustable gas block and a 8 or 9" barrel I don't remember exactly. I think there are a few company's making remans for the 300 now.

Has anyone thought of building a 7.62x39 SBR? Ammo would be super cheap and ballisticly similar to the 300. That could be a good alternative to non reloaders. 

I've read that the 7.62x39 isn't super reliable in the AR platform.  Nothing keeping you from doing it with an AK though....

[RangerTrace]

7 hours ago, NateTSU said:

My friend is using a factory carbine buffer with a adjustable gas block and a 8 or 9" barrel I don't remember exactly. I think there are a few company's making remans for the 300 now.

Has anyone thought of building a 7.62x39 SBR? Ammo would be super cheap and ballisticly similar to the 300. That could be a good alternative to non reloaders. 

I've read that the 7.62x39 isn't super reliable in the AR platform.  Nothing keeping you from doing it with an AK though....

[NateTSU]

I wonder if reliability issues are related to variations in ammo from commercial suppliers and poor QC from some surplus ammo. If it ran reliably it would be a cool and cheap alternative to the 300 for non reloaders. 

[jrswanson1]

43 minutes ago, NateTSU said:

I wonder if reliability issues are related to variations in ammo from commercial suppliers and poor QC from some surplus ammo. If it ran reliably it would be a cool and cheap alternative to the 300 for non reloaders. 

It has more to do with the magazines than the rounds.  If you get a lower that accepts AK mags, you're golden.  Otherwise, it's a crap shoot.

[NateTSU]

Oh I see, I would want an ar lower that uses standard mags so I could swap uppers. 

[steviesterno]

300 is a great caliber if you understand what it's trying to do and that fits what you need. It's not a magic, do all wonder round I don't think. 

It has been designed to run in a regular AR minus the barrel, so if you have an SBR lower already it's an easy upgrade. In 220 or so grain sub sonics, it's a pretty easy to suppress, thumper of a round. Like a 30-30 or 45 ACP it's heavy and slow, but you can use bullets from a 308 so better ballistic co-efficient. It's also really quiet since it's more like a big pistol round than a small rifle round. It has some drop, but inside 150 or so yards it's great. You can double that with a supersonic round, more if you're feeling squirrelly. If you reload, it's easy to make fancy loads, if you don't you can still get some decent rounds for under $1 each. It's designed to run in a short barrel, so you don't need to hang a can off a 16-20 barrel. it's a low pressure round, so not as sharp sounding even not suppressed, and an 8" 223 is terrible as far as blast, fire, and lack of punch. 

if you're building totally from scratch and like 9mm, the AR isn't the best platform for it. You could go sig, CZ scorpion, or other, and do an SBR off that. If i was totally starting from scratch, I probably would do an MPX. But I'm not, I have the AR lower. So a 300 upper is a couple hundred, not several thousand dollars. 

 

If you want a fun caliber to plink with, build a second upper in 22lr, like 5" barrel and a can. total hoot, cheap to shoot, fun for steel challenge, etc. Then throw the good upper on there when you get home. 

My goal was short as possible and still shoot, using an existing AR lower, mags, and port of arms, for under a grand. So for me, that was shorty 300 upper and a NEA stock. see the attached pic for how it looks with a 6" 22lr barrel. With the 8" upper I'm building in 300, it should be under 20" long but still pack rifle punch as needed. 

[StealthyBlagga]

Just be aware that it is possible to chamber and fire a .300BO round in a .223/5.56 rifle... at least once. Obviously this would be a bad thing. If you own AR15s in both calibers, you need to be really careful not to mix up the ammo.

[Nathanb]

Agreed. I don't bring out both sbr  uppers very often and when I do the other one and ammo goes away. 

Also this is an sbr dream rifle. From my 8.5 sbr to my 16 inch ruger ranch the velocity difference is about 60 fps. Compared to the hundreds of fps difference in 223. I believe the Saami spec and round was developed around an 8 inch barrel. I didn't even consider it until I got a can and registered lower