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Discussion Starter #1
My intent is to build a 300 Blackout pistol with an 8.5 inch barrel. As I'm perusing various websites to check barrels, and prices, I'm seeing things I don't like. I'm unwilling to jump through the hoops required to have an SBR, or suppressor. However, I prefer the heavier grain bullets, so the plan was to use, on average the 220 grain .308 bullet. The new Sig Sauer 220 Grain round has become my favorite.

Now several of the barrels I'm looking at indicate the use of supersonic rounds only, as suppressor rounds may not function, or lock back on the last round if there is no suppressor attached. Now I've seen 1.8 and 1.7 twist barrels, and wonder how much of an impact this would have on the function.

So to the collective expertise of the forum, does anyone have an experience with this?
 

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Rate of twist has more effect on the accuracy/Stabilization of certain bullet weights. 1 in 8 and 1 in 7 are very close. They would handle slightly heavier bullets. If you are looking to use lighter than average bullet weights I believe the 1 in 10 and 1 in 14 twist rates may benefit you more.

Barrel length and gas system would have more to do with the action functioning properly

That is just a quick reply. I am sure an expert will be in here soon to give a more detailed response
 
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This would certainly only be a gas issue. I'm not up on the particulars, but with that short of a barrel, you have a small window to get enough gas to operate the bolt. I'm assuming the 300BO does not have quite the pressure of .223/5.56, so it is more critical to load them hotter in a short barrel because they leave in such a short distance and dont have the time to build up the volume of gas needed.

If you are reloading, a fast powder/hotter load would help some of this problem. If you arent using a suppressor, you can load them supersonic(if load data allows) and not have a problem. Bullet weight isnt the direct issue.

The other way to address it would be to tweak the gas system to allow more gas, but this is something that needs to be looked at by a professional. The gas system is tricky and likely to be moreso in a small barrel.
 
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Adjustable gas blocks are available for the AR-15. I have a 10" 300 BO SBR and have not had a problem with 230 grain SMK, or Missouri coated cast bullets, using 1680. These are subsonic loads, and operate with or without the can. They also work on my 16" carbine, with or without the can. The 10" has a pistol length gas system, the 16" has a carbine length gas system. I believe the port on the carbine length is sized differently (larger). I don't have an adjustable gas block. My understanding is adjustable gas blocks may be necessary when using different powders with subs. Of course I also had to work up the load so that they would operate the bolt and lock back, in addition to accuracy testing, but all factory loads operated both uppers, no problem. I have not tried loading lighter weight bullets. You can do a web search to find out what size gas port you need for your gas system length, different manufacturers use different sizes.
 

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Discussion Starter #5
Adjustable gas blocks are available for the AR-15. I have a 10" 300 BO SBR and have not had a problem with 230 grain SMK, or Missouri coated cast bullets, using 1680. These are subsonic loads, and operate with or without the can. They also work on my 16" carbine, with or without the can. The 10" has a pistol length gas system, the 16" has a carbine length gas system. I believe the port on the carbine length is sized differently (larger). I don't have an adjustable gas block. My understanding is adjustable gas blocks may be necessary when using different powders with subs. Of course I also had to work up the load so that they would operate the bolt and lock back, in addition to accuracy testing, but all factory loads operated both uppers, no problem. I have not tried loading lighter weight bullets. You can do a web search to find out what size gas port you need for your gas system length, different manufacturers use different sizes.
The 1680 is my powder of choice, which gives me some hope! I thought about extending the barrel to a 10", and I'm going to a local store this weekend that specializes in AR's and hope they'll have some answers. The same place I'm picking up my Aero Precision Upper and Lower. There is a lot of knowledge in that store. I didn't think about an adjustable gas block. I have to see if they make pistol length fat boy gas tubes. (Just took a quick peek, and didn't see anything) Thanks for the idea on the gas block!
 

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Go with a 10.5 barrel/pistol gas.The round burns all powder in 9.5-10" so max effiecency is about 10.5.I shoot from 90gr to 240gr unsuppressed never had a problem.
 

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Discussion Starter #7
I agree with the 10.5" barrel. My store experts both said the adjustable Gas Block isn't necessary, but the extra couple of inches couldn't hurt. I've just invested in a Hornady single stage Lock-N Load reloading press, so the barrel will have to wait until closer to Christmas. That would make a fine gift to myself. Thank you all for your ideas, and advice.
 
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