Recommendations for 300 BO barrel length & twist rate.

[Jefe's Dope]

I have an extra lower I'm looking to build a 300 Blackout with. I'm looking to build a pistol/SBR. I'm in need of advice for a barrel length and twist rate. I'd be using a TB Ultra 7 as a suppressor shooting subsonics. This would be a general SD rifle/pistol with either a red dot or LVPO.

1. Barrel length?
2. Twist rate?
3. Chrome lined or not?
4. LVPO or Red dot?
5. Ammo recommendations?
6. I'm assuming I can use any of my Magpul 5.56 mags? Mods needed or recommended for these mags?
7. Am I missing something as I dont' know what I don't know.

 

[FuhQ]

I have an 8" pistol, a couple 10" pistols, and a couple of 16" .300 BLK rifles... The 10" is the best compromise. You don't gain anything over 10" barrels with subs, since it's a short-range cartridge, not designed for more than 100-200 yards tops. So, here's my opinions since I've been messing with this cartridge since I had to cut and form my own brass for it, when it was so new you couldn't even get factory brass or ammo for it... I still cut and form most of my own cases from 1x fired 5.56 NATO brass (the thicker necks from cut-down 5.56 brass is tougher, and lasts longer than factory thin-neck .300 BLK brass).

1.) 8" to 10.5" is ideal, since you gain virtually nothing over 10.5" of barrel with subs in a .300 BLK.
2.) 1:7 is fine for both lengths, you'll be good for up to 220gr subs.
3.) Chrome-lined is always a nice touch, but make sure you get one with 5R rifling like the Aero Precision CMV barrels.
4.) A good 1-6, 1-8, or 1-10 LPVO will be fine, or a fixed 3x or 5x like the Vortex Spitfire 5x Gen2 or Primary Arms Mini 5x.
5.) Roll your own for cheap range plinking. If you plan on hunting with subs, get some Hornady Sub-X 190gr ammo.
6.) Yes, you can use standard 5.56 mags
7.) Make sure you get a matching headspaced bolt from the same company that makes the barrels. This is always a good idea with any AR15.
8.) Yes, you're missing a .300 BLK...

 

[Bluedog82]

I had a 9” (the barrel length the round was designed to have a full powder burn in) and currently have a 6”. I would go 9” and under, makes for a noticeably more compact rifle than a 10.5 or longer 556 gun, even suppressed. My 6” with can is about as long as my 11.5 without the can. Red dot is totally fine and is what I’d recommend for most people. You WILL need 300blk specific mags for subsonics most likely, as they don’t always feed well in standard magazines due to the bullet shape.

 

[overmatch]

I have an 8" pistol, a couple 10" pistols, and a couple of 16" .300 BLK rifles... The 10" is the best compromise. You don't gain anything over 10" barrels with subs, since it's a short-range cartridge, not designed for more than 100-200 yards tops. So, here's my opinions since I've been messing with this cartridge since I had to cut and form my own brass for it, when it was so new you couldn't even get factory brass or ammo for it... I still cut and form most of my own cases from 1x fired 5.56 NATO brass (the thicker necks from cut-down 5.56 brass is tougher, and lasts longer than factory thin-neck .300 BLK brass).

1.) 8" to 10.5" is ideal, since you gain virtually nothing over 10.5" of barrel with subs in a .300 BLK.
2.) 1:7 is fine for both lengths, you'll be good for up to 220gr subs.
3.) Chrome-lined is always a nice touch, but make sure you get one with 5R rifling like the Aero Precision CMV barrels.
4.) A good 1-6, 1-8, or 1-10 LPVO will be fine, or a fixed 3x or 5x like the Vortex Spitfire 5x Gen2 or Primary Arms Mini 5x.
5.) Roll your own for cheap range plinking. If you plan on hunting with subs, get some Hornady Sub-X 190gr ammo.
6.) Yes, you can use standard 5.56 mags
7.) Make sure you get a matching headspaced bolt from the same company that makes the barrels. This is always a good idea with any AR15.
8.) Yes, you're missing a .300 BLK...

Would you say there is any detriment to going from 8" to a 7.5" barrel? Asking for my own bolt gun build I'm currently planning out. Planning a 1:7 twist.

 

[FuhQ]

Would you say there is any detriment to going from 8" to a 7.5" barrel? Asking for my own bolt gun build I'm currently planning out. Planning a 1:7 twist.

If you’re going less than 8”, I’d go 1:5 twist.

 

[overmatch]

If you’re going less than 8”, I’d go 1:5 twist.

Considering I'll be direct thread mounting a 9.5" can to the end of it (that's now discontinued), do I have any more risk of bullet separation?

 

[FuhQ]

Considering I'll be direct thread mounting a 9.5" can to the end of it (that's now discontinued), do I have any more risk of bullet separation?

I’d check with the particular bullet manufacturer, then have them email it to you, so you have things in writing. Regardless, I’d get an alignment rod if you don’t have one, and check alignment before shooting it.

 

[overmatch]

I’d check with the particular bullet manufacturer, then have them email it to you, so you have things in writing. Regardless, I’d get an alignment rod if you don’t have one, and check alignment before shooting it.

What are the downsides to going 1:7 at 7.5”

 

[Bluedog82]

What are the downsides to going 1:7 at 7.5”

I don’t see any. My 6” is a 1:7 and I’ve shot 220gr subs just fine.

 

[FuhQ]

What are the downsides to going 1:7 at 7.5”

My 8" .300 BLK pistol has a 1:7 and it shoots great at 100...Headshots all day long. And you'd most likely be fine at 7.5". But for me personally, if I was going below an 8" barrel, I'd want to drop down to a 5" twist just to pickup the RPM's since it spends much less time in the barrel.

 

[overmatch]

I don’t see any. My 6” is a 1:7 and I’ve shot 220gr subs just fine.

Exactly the datapoint I was looking for. Thank you!

 

[J. W.]

Running a 8” 1:8. It stabilizes everything I’ve tried so far, but the Lehigh 194 ME bullets do leave a slight “swipe mark” on the holes at 100 yds, so maybe 1:7 would do a little better.

 

[fanninland]

Timely thread here for me. No previous BO experience. Acquired a used 10.5” Noveske barrel which came with their standard pinned gb. For shooting subs suppressed any idea if agb is usually needed? (have’m on all my 5.56/6.8 sbrs)

 

[FuhQ]

Timely thread here for me. No previous BO experience. Acquired a used 10.5” Noveske barrel which came with their standard pinned gb. For shooting subs suppressed any idea if agb is usually needed? (have’m on all my 5.56/6.8 sbrs)

None of mine are adjustable...They work just fine.

 

[Bluedog82]

Timely thread here for me. No previous BO experience. Acquired a used 10.5” Noveske barrel which came with their standard pinned gb. For shooting subs suppressed any idea if agb is usually needed? (have’m on all my 5.56/6.8 sbrs)

Same as @FuhQ, don’t have one on mine either.

 

[alamo5000]

1. Barrel length?

Your choice.

2. Twist rate?

1/7 will stabilize any 30 cal bullet made. 1/8 is still ok.

3. Chrome lined or not?

Chrome lined is absolutely useless in this context. It will not add anything at all that could even potentially help you.

If you get a $150 barrel and you shoot $20,000 worth of ammo through it you can probably afford to buy another $150 barrel.

4. LVPO or Red dot?

Red dot. For sure. An LPVO will be heavy and will not bring much to the party. Then again you might be trying to do something weird like shooting subs out to 4 or 500 yards...

5. Ammo recommendations?

Reload your own. Factory ammo is expensive. For good quality heavy subs are very expensive.

6. I'm assuming I can use any of my Magpul 5.56 mags? Mods needed or recommended for these mags?

The only reason to use the 300blk marked mags is that they will handle some of the big fat bullets better. Sometimes with certain bullet shapes they can bind in standard mags but not always. The official answer to this is definitely dependent on the specifics of the ammo.

7. Am I missing something as I dont' know what I don't know.

Define what you SPECIFICALLY want to do and build accordingly. Your basic concept needs to be sharpened up a whole lot. You need to get much better idea of how you plan to use the rifle BEFORE you start buying parts.

The questions you posted and how you framed it says you have not thought through what your end product should be.

 

[45-90]

Get a 16" barrel & 7 twist , no hassles. Forget the suppressor...
And it's not true there is no gain past 8 "...that is a myth.
"Mr guns and gear" has a whole program on 300 Blackout vs barrel lengths...check out the video.
Even the 9mm gains from a 16" barrel...used to have a 5.5" very slow, a 10" faster, and the 16" was even faster. The pinned 14.5 " is slower than the 16" with the 9 mm!
In the 300 Hamr the 11" was much slower than the 16".
Done with pistols, and short barrels, put on hearing protection and enjoy the 16" ...forget the weight, expense, and length of the suppressor, and the legal hassles of SBR and suppressors.
But do it any way ya want,....your gun...all those extras & accessories add to the economy.

 

[Bluedog82]

Get a 16" barrel & 7 twist , no hassles. Forget the suppressor...
And it's not true there is no gain past 8 "...that is a myth.
"Mr guns and gear" has a whole program on 300 Blackout vs barrel lengths...check out the video.
Even the 9mm gains from a 16" barrel...used to have a 5.5" very slow, a 10" faster, and the 16" was even faster. The pinned 14.5 " is slower than the 16" with the 9 mm!
In the 300 Hamr the 11" was much slower than the 16".
Done with pistols, and short barrels, put on hearing protection and enjoy the 16" ...forget the weight, expense, and length of the suppressor, and the legal hassles of SBR and suppressors.
But do it any way ya want,....your gun...all those extras & accessories add to the economy.

Everyone is entitled to their opinion. I personally think that’s a silly way to go and mostly negates a large part of what makes 300blk unique.

 

[fanninland]

 

[45-90]

Yep, and I have alot of opinions...which sometimes change. I do not have any AR pistols after the last federal fiscal. It was an easy decision, cause I wasn't going suppressor, and the short barrel had more negatives than positives...mostly just looked cool..

Shot thousands of subs in various calibers...
300 blackout is not unique but common place.
Here is more unique, a 220 SMK 300 blk, vs a 750 gr 510 whisper.
Or a 300 Blackout that actually mushrooms at subsonic velocities, the Speer varmint does not.
A 9mm luger over 2600 fps from a 16" barrel AR.
A 185 gr 9mm luger, or shotgun primers in a 308 case.
Making a die to take 30-06 to 8.6 blackout, one shot without case lube, but just a light spray makes it easier and keeps tbe cases from getting hot, while sizing the brass.
Making 6mm ARC out of 308 Win or 35 Rem out of 308 Win.
18" AR 200 gr SMK at 2673 fps. Or s 230 SMK at 2620 from a 308 bolt gun...and will be improved up on. Then there is hybrid cases reformed to other calibers.
When in the experimental mode...almost nothing is left in its original condition, except no barrels under 16" for me anymore. I like performance of the longer barrels ...my 308s run from 16" to 30". Just got done loading done subs for 45-70 & 45-90...so that might be pretty unique...but almost everyone has or has tried the 300 blackout, it really quite common, with many variations...the way they like it I presume.

 

[91Eunozs]

A lot of what I’d say has already been covered, but one additional consideration is magazine selection. I use Lancer 300BLK-specific mags almost exclusively for my 300BLK uppers, and try very hard to not use Lancer mags for anything else, even though I love them. the 300BLK specific mags have the internal ribs trimmed down to allow better stacking for the heavier 30 cal bullets.

That said, I’ve never had any issues feeding 300BLK from decent quality GI mags (aluminum and steel), or standard Magul mags. All my 300BLK barrels have standard M4 cuts on the barrel extension too.

I want there to be a tactile difference when I grab the mag…just in case I left a Magpul, or GI mag in my plate carrier or they got stacked in the wrong safe by accident, etc. Way too easy to feed a 300BLK round into a 5.56 upper, or vice-versa, and I like having one more mechanism that won’t negatively impact performance, but helps me prevent that occurrence.

—-

Re: your list, I have several 300BLK flavors that each fill their own unique niche, but for an SBR here’s my recommendation:

1. Barrel length? 8-9”
2. Twist rate? 1:7, no slower than 1:8
3. Chrome lined or not? Doesn’t really matter, but I prefer chrome-lined
4. LVPO or Red dot? Red dot for HD; you can always add a magnifier if you want to stretch its legs
5. Ammo recommendations? Roll your own, but I really like the Barnes 110gr black tips for supersonic loads on pigs, the Hornady 110s do well too, and are a lot easier to find in my AO. I’ve gravitated to the Hornady sub-X bullets for subs. Seem to work well for me.
6. I'm assuming I can use any of my Magpul 5.56 mags? Mods needed or recommended for these mags? SEE ABOVE
7. Am I missing something as I dont' know what I don't know. I’d recommend an adjustable gas block. Not really needed, but easier to tune when switching between supers and subs a lot. I’ve gotten to the point where some uppers are used almost exclusively for subs, and others for supers, but nice to have an easy mechanism to tune without swapping springs, buffer weights, etc.

Also, do what works for your needs, but I’m at the point for 300BLK where I zero my subs at 25 yards, and my supers at 50. I have one 16” upper that is zeroed at 100 yards…it has an LPVO and micro red dot on an offset mount in case something pops up close.

300BLK is a hoot to tune and shoot…so easy to tune! And a blast shooting subs suppressed.

HAVE FUN!