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300 BO Short barrel build & use - thoughts?

Smitty192

Smitty192

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Feb 23, 2017
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I'm contemplating a 300 BO short barrel build. I've got enough spare parts laying around so cost of entry won't be bad. I just need a barrel.

For reference, I've got a 12" 556 that I really enjoy, but what I don't have is a very compact weapon that works well with my suppressor and would be reliable and ultra quiet. My initial thoughts would be a 5" or 8" barrel running subsonic loads only. The subsonic ammo certainly has it's limitations and realistically this is a 200 YD platform, maybe less.

Outside of it being compact and quiet, what practical use does it have? I'm curious to everyone's thoughts and opinions on the practical application of a subsonic 300 BO compact platform. Good, bad and otherwise.

5" or 8" barrel? Performance should be the same, I think. I've read a lot about 1:5 twist for short barrel, heavy projectiles. Has anyone gone that route?

Any particular concerns with this type of set up as it relates to interacting with a suppressor? Any other general thoughts and opinions on the matter would be appreciated. And as always...snide comments are welcome.
 
Just finished building my .300blk SBR not too long ago. Built it so that it could serve where needed i.e. HD, truck gun, range toy, etc.

I went with an 8.3" BA Hanson barrel 1:7 twist. Unfortunately when I started my build I wasn't aware of anyone offering a 1:5 twist barrel or else I might've gone that route, some of the videos showing the ballistic performance with the faster twist rate seem promising, MAC on youtube has a video comparing a 1:5 to a 1:7 or 1:8 shooting at watermelons and the difference in energy transfer between the two is huge.

I keep a YHM Resonator R2 mounted on the end via Dead Air Keymo and haven't had any issues with that setup. Decided to try out the Bootleg Inc. adjustable BCG as opposed to an adjustable gas block. Haven't got enough rounds through it to definitively say whether or not it's an advantage but I do like how easy it is to use.

Also went with a side-charging upper to completely eliminate the possibility of gas to the face, am a fan of that choice.

273772439_4927753317309615_2932671476674133886_n.jpg
 
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Maverick3009 said:
Just finished building my .300blk SBR not too long ago. Built it so that it could serve where needed i.e. HD, truck gun, range toy, etc.

I went with an 8.3" BA Hanson barrel 1:7 twist. Unfortunately when I started my build I wasn't aware of anyone offering a 1:5 twist barrel or else I might've gone that route, some of the videos showing the ballistic performance with the faster twist rate seem promising, MAC on youtube has a video comparing a 1:5 to a 1:7 or 1:8 shooting at watermelons and the difference in energy transfer between the two is huge.

I keep a YHM Resonator R2 mounted on the end via Dead Air Keymo and haven't had any issues with that setup. Decided to try out the Bootleg Inc. adjustable BCG as opposed to an adjustable gas block. Haven't got enough rounds through it to definitively say whether or not it's an advantage but I do like how easy it is to use.

Also went with a side-charging upper to completely eliminate the possibility of gas to the face, am a fan of that choice.

View attachment 7907132
Click to expand...
A couple of things stand out to me. I watched the MAC video this morning and didn't catch the different twist rate. There was a substantial difference in the 5 twist. Never thought about a side charging upper to eliminate blowback, interesting. Maybe an 8" barrel with 1:5 twist would be a solid combination. Nice set up. Very close to what I have in mind. Looks like it folds up to a very concealable package. Thanks for the input.
 
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Folds up small enough to fit into my Vertx Gamut or Eberlestock Switchblade. The LAW folder mixed with the side-charging upper made it a little tricky. The stock just barely contacts the charging handle when folded but not so much where I'm concerned about any extra stress or wear being put on either part.

290949724_448863453391630_4431166964915905983_n.jpg
 
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They're fun. My original thought was that it'd make a great short to medium range do-all rifle in a compact package.

In retrospect, everything's a compromise. The additional ammo cost even when reloading has me reaching for the 5.56 most of the time. I have a dillon 750 set up to convert to .300BO on the press too. If I could go back and do it over, I'd have bought an mpx or something similar in 9mm for uber quiet plinking instead.

It wasn't so bad when the 125gr TNTs were cheap and plentiful. Coated projectiles don't work well in the progressive for subs. The coating gets shaved off when seating. I can't figure out a way to make it economical to shoot in volume, especially with subs.

If I'm missing something, please, for the love of God let me know.
 
vinniedelpino said:
They're fun. My original thought was that it'd make a great short to medium range do-all rifle in a compact package.

In retrospect, everything's a compromise. The additional ammo cost even when reloading has me reaching for the 5.56 most of the time. I have a dillon 750 set up to convert to .300BO on the press too. If I could go back and do it over, I'd have bought an mpx or something similar in 9mm for uber quiet plinking instead.

It wasn't so bad when the 125gr TNTs were cheap and plentiful. Coated projectiles don't work well in the progressive for subs. The coating gets shaved off when seating. I can't figure out a way to make it economical to shoot in volume, especially with subs.

If I'm missing something, please, for the love of God let me know.
Click to expand...
When it comes to quietly plinking, the 300 is pricey for sure.

9mm subs are actually pretty economical to load.
I run plated 147gr Xtremes.
 
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vinniedelpino said:
If I could go back and do it over, I'd have bought an mpx or something similar in 9mm for uber quiet plinking instead.
Click to expand...

I did have a CZ Scorpion I'd built up which was awesome w/ 147gr and heavier rounds, shot like a dream.

However, I'm a glutton for punishment so when a deal popped up for a 1cm PCC I had to jump on it. Why am I like this?

291368164_892702195455291_3223963372626092719_n.jpg
 
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My vote for the OP's question would be to go for the 8" barrel. While the 5" would definitely do subsonic, the 8" would give you the added option of decent performance with supersonic rounds if you decide to use them.

The gas system on the 8" would be less finicky, and the 300BO loses less performance at 8" than a 5.56/.223 at 8" thanks to bullet weights and powder burn rates.

The Hornady 190 Sub-X is a great option for subsonic 300 BO that is pretty accurate with at least some expansion.

Pics are a 300 BO with an 8" barrel plus about a 2.75" muzzle brake. Just the handguard section is 7".
 

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I really like my 300 BO, but the rule I have for myself and subs is “if you wouldn’t shoot it with a pistol, don’t shoot it with subsonic 300BO.”

Even if the heavy bullets just pencil through, a 30 round mag lets you make a lot of holes.
 
I love my custom .300 BLK that I built many years ago, back around the time it first came out as a SAAMI spec cartridge (IMO, the only good thing Kevin B. ever did). It was previously a wildcat from the late 80's/early 90's known by several names like .300 Whisper, .300 Fireball, .300-221 Fireball, 7.62x35mm, etc... but it was never standardized until 2010.

It's a great round, and the fact it uses minimal amounts of magnum pistol powder shows just how efficient it really is. My 220 sub load uses 9-10gr of powder and a magnum SR primer. I cut down and make my own brass (super simple). And my cost of reloading is about $0.35 each for my subs using Berry's 220 Spire Points. Roughly $0.25 per bullet, $0.05 per primer, and $0.03-$0.05 for powder per round (depending on type). Brass is free, and once I bought the tools to make my brass, I wrote that expense off in my mind after making the first thousand pieces with the money I saved using free range pickup brass.

As for those saying it's useless, it's definitely not. It works great with both super and subs within reasonable limits. A lot of folks hunt with .300 BLK with supers...Even deer. Some folks are using expandable PD subsonic ammo for hunting. Personally, I don't shoot supers out of mine, but that's because I have suppressors. What makes .300 BLK so much fun is just how quiet the subs really are.

I like it so much, I'm fixing to build another one, unless I get a good deal on an already built upper with the features I like, for less than what I can build one for. I normally built all my own AR's, but these days technology has allowed high-quality and cost efficiency to run fairly close. For example, I never disliked Aero Precision, I just always went with top-tier components in my rifles, and not so much budget friendly options... So I never really gave them much thought. A buddy of mine had just bought a new 10" .300 BLK Aero upper, and brought it to the range. After letting him use my suppressor (while I was sitting right there next to him) and letting him use some of my precision-loaded 220 subs, he sighted in at 100, and then started hitting the 5" gongs at 200 4/5 times. I might end up getting one of those uppers myself, as I have a few stripped lowers sitting around.

My point being, if you can shoot worth a damn, and make good ammo, the .300 BLK can do whatever you want it to do, within reasonable distances.
 
reubenski said:
I guess 300BO subs do have a use. You can plink with them and tell people about it on the internet.
Click to expand...
I’m going to drag mine up into my bow blind for general deer season this year…
 
Maverick3009 said:
Just finished building my .300blk SBR not too long ago. Built it so that it could serve where needed i.e. HD, truck gun, range toy, etc.

I went with an 8.3" BA Hanson barrel 1:7 twist. Unfortunately when I started my build I wasn't aware of anyone offering a 1:5 twist barrel or else I might've gone that route, some of the videos showing the ballistic performance with the faster twist rate seem promising, MAC on youtube has a video comparing a 1:5 to a 1:7 or 1:8 shooting at watermelons and the difference in energy transfer between the two is huge.

I keep a YHM Resonator R2 mounted on the end via Dead Air Keymo and haven't had any issues with that setup. Decided to try out the Bootleg Inc. adjustable BCG as opposed to an adjustable gas block. Haven't got enough rounds through it to definitively say whether or not it's an advantage but I do like how easy it is to use.

Also went with a side-charging upper to completely eliminate the possibility of gas to the face, am a fan of that choice.

View attachment 7907132
Click to expand...
That looks great, what is the side charging upper?
 
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Smitty192 said:
I'm contemplating a 300 BO short barrel build. I've got enough spare parts laying around so cost of entry won't be bad. I just need a barrel.

For reference, I've got a 12" 556 that I really enjoy, but what I don't have is a very compact weapon that works well with my suppressor and would be reliable and ultra quiet. My initial thoughts would be a 5" or 8" barrel running subsonic loads only. The subsonic ammo certainly has it's limitations and realistically this is a 200 YD platform, maybe less.

Outside of it being compact and quiet, what practical use does it have? I'm curious to everyone's thoughts and opinions on the practical application of a subsonic 300 BO compact platform. Good, bad and otherwise.

5" or 8" barrel? Performance should be the same, I think. I've read a lot about 1:5 twist for short barrel, heavy projectiles. Has anyone gone that route?

Any particular concerns with this type of set up as it relates to interacting with a suppressor? Any other general thoughts and opinions on the matter would be appreciated. And as always...snide comments are welcome.
Click to expand...
Ultra quiet is subjective…. Dependable accuracy up to 100yds.
 
Maverick3009 said:
Unfortunately when I started my build I wasn't aware of anyone offering a 1:5 twist barrel or else I might've gone that route, some of the videos showing the ballistic performance with the faster twist rate seem promising, MAC on youtube has a video comparing a 1:5 to a 1:7 or 1:8 shooting at watermelons and the difference in energy transfer between the two is huge.
Click to expand...
Do you have a link to that video?
I’ve tried to find it.
Thanks
 
Maverick3009 said:
He's actually testing a 5.5" Sig Rattler vs a 9" BCM Barreled AR so it's probably harder to find because he doesn't specify the twist rates in the title of the video.

Click to expand...

I know it's a small sample, but the shorter barrel and 5 twist combination seems to do well with the supersonic rounds too. This was a big question for me. If I wanted to run supersonics rounds, would the 5 twist work well.
 
I’ve done 300blk in 6”, 8” and 16”. All are 1:7 twist

The Barnes 110 gr blacktips will give reliable expansion velocity out to a couple hundred yards if I recall out of the 6” 1:7. The minimum expansion velocity for the 110gr Barnes blacktip is down around 1300-1350.

If you want a reliable supersonic trunk/truck gun for 0-200 yards it will work well. Add a folding stock like a previous poster here did and it is uber short.

Mine was 100% reliable with supers with and without a suppressor. Subs were 100% with a can and about 75% without a can. I didn’t try messing with it and sold it to a demanding co-worker who had to have it.

I’ll try to attach a graph of velocities with the Barnes 100gr and different barrel lengths…

300BLK v 556 Trajectory.png
 
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