SBR 300 BLK tips

[Scratch]

I'm thinking I want to do an SBR and I've been wanting to try a 300 Blackout project, so I thought I might as well combine them. I'll probably go the trust route and am getting that process started, but I thought I'd start my research now. I'm not a big fan of Arfcom so I'm hoping I can get some questions answered here instead.

What twist rate should I go with? I've heard there can be cycling issues with short barrels for 5.56. Is it the same for .300 BLK? I want to go maybe 7-8 inches...

Im planning on building it as a pistol first, so I can some shooting in, then once the PW comes back, switching out the buffer tube and adding a stock. Also may add a suppressor to it at some point.

Any other tips...?



ETA: feel free to post any pics of your SBR to get me motivated....

[Shawski]

Mine is a 1:8 10.5" with a pistol length gas system. It's been flawless so far after 500+ rounds. A 7-8" barrel shouldn't be a problem as it was designed from the beginning to run that short of a barrel. Are you going to be reloading for it as well?

[Scratch]

Yes, I'll be reloading for it also, so I know I'll be able to tune my loads for it, just wanted to see what most people are using for twist rates, and if they've run into any problems.

I may also get a same length 5.56 barrel for it, since I already have so much of it so if anybody has any input on shorter length 5.56 barrel twist rates... Let me know.

[Shawski]

Biggest problems I've heard about is feeding. When you're making ammo for it you want the part of the bullet that measures .250" at the front rib on the side of your mags. And try using either different colored mags or plastic/metal mags for .300 BLK and 5.56 so you don't get them mixed up.

I also have a 10.5" 5.56 SBR with a 1:7" twist barrel on it, no problems so far and it's pretty accurate.

[Scratch]

?... you want the part of the bullet that measures .250" at the front rib on the side of your mags...

Huh.... I'm not getting that all.....?

[Shawski]

The neck on a 5.56 round sits against the side rib in a mag, so the front of the round bumps out to the center when it feeds. Somewhere between the tip of the bullet and where it measures .308" it measures .250" with a .300 BLK. That's where you want the bullet to ride against the rib in the mag so they feed straight into the chamber and don't bounce around or get snagged up. I'll see if I can find a picture later if you'd like.

[Scratch]

The neck on a 5.56 round sits against the side rib in a mag, so the front of the round bumps out to the center when it feeds. Somewhere between the tip of the bullet and where it measures .308" it measures .250" with a .300 BLK. That's where you want the bullet to ride against the rib in the mag so they feed straight into the chamber and don't bounce around or get snagged up. I'll see if I can find a picture later if you'd like.

Yes to pics please... Are you talking about adjusting the OAL to get to this measurement?

[rugersol]

IIRC, mine's a 9in ... forget the twist ... thought they were all the same?

I wanted to shoot both subs & supers ... so I put an adjustable block on ... works great!

[JFettig]

I'm probably one of the very few who have successfully made sub 10.5" barrels. There is a secret to making them work with subs. If you're shooting supersonic ONLY you can run them. I have never run supersonic bullets through my 7.5" barrels, only subs.
Here is one of my 7.5" barrels with a 6" can, shorter than a 12.5" SBR


I have barrel blanks and everything needed to make more, just need to find time. Let me know if you want one.

I have always used 1:8 blanks and it has been the standard for a long time. Some have run 1:7 and 1:6 barrels. I have only heard issues with really short 1:8 barrels and 240gr+ bullets.

[Shawski]

Yes Scratch, you would adjust your OAL to fit the mags. Not always possible with the shorter, lighter bullets. Anyways, here's a picture to help me explain:
[img][/img]

[Scratch]

Aahhhh... I get it now shawski. Seems like a small hassle to run 300 BLK and another hassle to run SBR 5.56....


Maybe I should just SBR my AK...

[FJ540]

Some guys file the ribs out of pmags, some load short to compensate, and I've had no issues loading to full mag length with SMK's in the 150gr class. As long as the ogive doesn't press the nose of the bullet into the front rib (pmags), I haven't found any feed issues and haven't de-rated mag capacity either. I get better performance out of the loads with less free case capacity, so I don't load them that long in regular practice, but I have done it.

Some guys seem to have issues with weak mag springs not being able to lift a full stack of heavy subs, so they've gone as far as cutting down pmags since the 20's weren't available (they are now) and using the 30rd springs for extra lift oomf.

Sub loads tend to be an issue of gas port size, buffer weight, and powder used. Some powders just don't produce enough gas to cycle the system. Too small of port with too heavy of buffer will also cause short stroking issues. It's not overly difficult to get these guns running both subs and supers. I'm running a .100 port on a 7.5" barrel (1:10 twist - not ideal), but haven't played with subs much yet. The Hornady book load using h110 worked fine with the 225gr bullets I tried and a carbine weight buffer and m16 carrier.

[JFettig]

Jim, you'll probably find that you won't get subs to cycle reliably even with a 1/8" gas port on your 7.5"

I had to custom make my gas system. My original 7.5" 300WTF has 2 plugged gas ports

[FJ540]

Except I've already done it.

The only question is whether or not they remained subsonic. I was pushing some mid 800's in the carbine, and those wouldn't cycle the pistol - but the hotter 225's ran the pistol only I never clocked them.

[FJ540]

I should also add - I stopped opening the gas port on the pistol when it reliably functioned with my super loads. I don't intend to run subs out of that unless MN loses the idiot regs or I end up with a O2 SOT.