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-   -   10.5" 300 AAC Blackout SBR (http://m14forum.com/m16-ar15/146001-10-5-300-aac-blackout-sbr.html)

jason777 September 13th, 2013 02:19 AM

10.5" 300 AAC Blackout SBR
 
Hi,

I'm taking the SBR plunge soon. I'd like to build a 10.5" with an A2 upper and std front sight post. With a pistol length gas tube, would that put the sights too close together to zero at 25 yards?

I really hate the flat top 300 Blackout uppers I've seen with flip up sights, so I'm trying to build something more like the original A2.


Thanks.

Clanjf September 13th, 2013 05:52 AM

Why not build it with carbine length gas system? From what I understand, the longer the gas system, the more reliable the weapon.

Jgun September 13th, 2013 06:20 AM

Mine is second hand information, but from what I've been told and have read, The SBR 300 BLK's cycle more reliably with a pstol length gas system. If you were to go to a 14.7" (16" with muzzle device), the carbine gas system length is fine but I think your making the correct decision to go with pistol length for a SBR.
BTW, Just a thought but if you have a full length hand guard, you could go with a set of the fixed sights that Larue sells, instead of the folding BUIS. I think you may be limiting yourself if you go with an A2 fixed front sight, and unless you use a handguard that allows you to remove it without removing the front sight (2 piece, non FF ?) you will be forced to remove the front sight if you want to remove the gas tube or handguard. Don't those front sights require that the barrel be drilled for those two lower dowl pins that hold them on?

GSO September 13th, 2013 07:47 AM

Quote:

Originally Posted by Clanjf (Post 1213628)
Why not build it with carbine length gas system? From what I understand, the longer the gas system, the more reliable the weapon.

Not necessarily true, particularly for 300BLK. I've got a 9" AAC upper with pistol gas (on a pistol lower until my stamp comes in) and a home-built 16" carbine with DSA barrel in 300BLK. If the gas port is too small on a carbine gas system, it will not be reliable, particularly when shooting subsonic ammo. The DSA barrel gas port is 0.120", which works well for all factory ammo I've fired. Robert Silvers, from AAC, and designer of the caliber, recommends a 0.120" to 0.125" for carbine gas.

As far as the original question, I guess the issue is whether the sight radius is too short to zero at 25 yards. I couldn't state for sure one way or the other, but I would imagine you could get close. I don't have one handy, but from memory, height of the sight plane above the bore should be about 2" or 2-1/2". This, in my mind, is certainly "good enough for government work" at that distance.

I personally would go with the flat top and put an EOTech on top. There is one designed for 300BLK, but any would work great at that distance.

You can get a lot more info on the 300BLK at:

http://www.300blktalk.com/forum/index.php

DaJeepGuy September 13th, 2013 07:51 AM

Quote:

Originally Posted by GSO (Post 1213686)

I personally would go with the flat top and put an EOTech on top. There is one designed for 300BLK, but any would work great at that distance.

You can get a lot more info on the 300BLK at:

http://www.300blktalk.com/forum/index.php

I just got the EOTech that was designed for the 300 BLK and put it on my Daniel Defense. The reticle is a little thick, but at the distances that the rifle will be used it really isn't a factor.

jason777 September 13th, 2013 08:31 AM

Thanks for the replies. From info on another forum, any barrel 16" or under needs a pistol gas system to produce enough pressure to reliably cycle a 300 BLK, or like you said, a bigger gas port. I'll be sending off my paperwork early next week and I've never even heard of anyone building an A2 upper 300 BLK SBR, so I'm skeptical about becoming the guinea pig LOL.

I'd love to just put a short barrel on my 5.56, but I don't want to blow out my ear drums and eye sockets if I had to shoot in closed quarters. With a .30 cal round moving around 2,000 FPS, It would be perfect for a home defense weapon if I had to shoot it indoors and it would have better range and accuracy than a handgun.

cl60cruzer September 13th, 2013 09:46 AM

have you seen the new .458 Winchester magnum snub nose?

GSO September 13th, 2013 11:35 AM

Quote:

Originally Posted by jason777 (Post 1213715)
Thanks for the replies. From info on another forum, any barrel 16" or under needs a pistol gas system to produce enough pressure to reliably cycle a 300 BLK, or like you said, a bigger gas port. I'll be sending off my paperwork early next week and I've never even heard of anyone building an A2 upper 300 BLK SBR, so I'm skeptical about becoming the guinea pig LOL.

I'd love to just put a short barrel on my 5.56, but I don't want to blow out my ear drums and eye sockets if I had to shoot in closed quarters. With a .30 cal round moving around 2,000 FPS, It would be perfect for a home defense weapon if I had to shoot it indoors and it would have better range and accuracy than a handgun.

A 16" barrel with the proper gas port size can use a carbine length gas system. I know that AAC and DS Arms both are properly built and function with any factory load. Any shorter than 16" and you do need the pistol gas though.

RiskyBiscuit September 17th, 2013 02:51 PM

I'm putting a UBR stock (w/Carbine spring/buffer) on my 10.5" 300BLK upper. We'll see how it goes. It'll run suppressed w/Subsonic most of the time.

Klem September 21st, 2013 08:44 PM

I'm a Blackout shooter.

There is a big difference in barrel pressures at the gas ports between 5.56 and .300BLK. The larger diameter barrel allows for chamber pressure to drop quicker so you need to harness that pressure early.

I recommend a pistol length gas block in all blackout lengths. 16" barrels are now being marketed with pistol length bas ports. Use a standard light buffer. Gas pistons do not work in a 300BLK as they require more gas than gas impingement. The point being 300BLK is almost under-gassed.

A traditional triangular Forward Sight Base at the 4" mark (pistol length gas port) is impractical. Use a stock that extends over the gas port to the muzzle. It protects it and will only be about 300gms. Put a foresight on the top rail as far forward as it can go. If you don't like folders then use a fixed system designed to go on a Pickitinny like the fixed 'Front/Rear Combo' marketed by Daniel Defense (about $120 per set).

That said, I hope you realise that a 10" barrel will give you starting velocities below 2,000fps. 1,800-2,000fps is the threshold velocity for hunting rds to do their thing. Your ballistic tips will act like solid lumps when impacting flesh. Blackout and SBR do not go well together for a practical rifle. But, if you want to look cool and punch holes in paper it's perfect!

Klem September 21st, 2013 09:05 PM

Quote:

Originally Posted by RiskyBiscuit (Post 1216251)
I'm putting a UBR stock (w/Carbine spring/buffer) on my 10.5" 300BLK upper. We'll see how it goes. It'll run suppressed w/Subsonic most of the time.

OK, just read your post above.

While 10.5" is a woeful choice for supersonic it is an excellent choice for subsonic.

Dedicated subsonic hunting projectiles and ammo is hard to find. I have only found two manufacturers; LeHigh and Outlaw State. Everything else acts like a solid lump when it hits flesh. Fantastic penetration, but doesn't do anything more than a crush-trail. Hollow pt pistol rds do more damage than a bullet designed for supersonic being used subsonic. Interestingly, that's how the Blackout is marketed, by turning a weakness into a strength. "Look at the fantastic penetration, 15 jugs of water!" (Translation: over-penetration and wasted energy, jugs of water that barely flinch and leak out slowly...no temporary wound cavity, and all because a big projectile is going too slow to deform).

But again, if all you are going to use an assault rifle for is recreation at the range...cheapest ammo that works I guess.


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