Building a 300 black out SBR dedicated subsonic gun... twist rate?

[SuburbanHick]

So I might be over thinking this and trying to re invent the wheel. But the heavier the bullet the more twist it requires to be stable. With that being said if I'm running 200+ grain subsonic ammo out of a ar15 with a 10 inch barrel, wouldn't it be best to go with the fastest twist I could find? I can BHW offers 1-7, X-cal offers 1-6.... several options for 1-8....

the main question I have to ask is if I were looking to get the most accuracy out of it, the bullet would be more stable at a faster twist rate... would this theory be amplified by being subsonic?

Ive used bergers stability calculator and the numbers and theory line up (eventho 1-6, 1-7 and 1-8 twist sufficiently stabilize a 200+ grain pill) ..

 

[Sierra770]

I have used 1-7 and 1-8 without issue. I used 220smk and 208 AMAX.

 

[Cornholeo]

I'm using 175SMK and 220RN in 1:7 without any problems, 8" barrel

 

[blbennett1288]

I use a Ballistic Advantage 8.3" 1:7 with Sigs 220gr OTM elite and it groups great

 

[Former0302]

I have a 1:7 twist and it's a great shooter from 110 gr AMAX's to 220 gr SMK's and everything in between.

 

[spife7980]

You are correct, the heavier the bullet the more twist will be required to keep it stable. The slower it is the more twist will be required. Both heavy and slow will need even more twist. I dont have a semi but I do have a bolt and I run subsonic 220 round nose sierras with a 1:7 twist and they stabilize great. 1-7 seems to be the standard for heavy subsonics.

 

[Hairy Biker]

The whole point of conventional heavy bullets leaving the barrel at 1050fps is that they should exist on the ragged edge of stability. They need to be stable in flight but then turn rapidly unstable upon hitting an object.

Take the 240 grain Sierra Matchking. When it hits a target, to get best effect, you want it to start tumbling within an inch. The ideal exit hole should look like a knife wound. I have had 240s pull internal material out on their journey.

This was unlike the 210 Berger which was too stable and ended up producing multiple needle point holes.

So 1/8 is fine. There has been a tendency to move towards 1/7 and I can see the logic for flight stability given it should/should make the bullet even more unstable when it hits. Also the modern subsonic specific bullets with petals and huge hollow points would seem to require a "supersonic" approach to use where "subsonic" loading is more like black powder cartridge.

At supersonic, I have used 125 grain Nosler BTs out to 700 yards with good effect but struggled with mid range weights. Groups do not seem to close up again until hitting the 180 grain mark.

 

[David4570]

AAC designed it's 9" PDW with a 1/7 twist and it runs FA with super/mid/sub weight(with can) rounds. I think they knew what they were doing.

 

[Hairy Biker]

Adesigned it's 9" PDW with a 1/7 twist and it runs FA with super/mid/sub weight(with can) rounds. I think they knew what they were doing.

Because a 9 inch barrel benefits from a faster twist for longer range stability. But the can would be holding in a lot of powder burn tha would be better consumed in the barrel.

It it is all a game of trade offs.

 

[David4570]

Just how much barrel does a subsonic round need to complete its powder burn? Specifically the 187gr. Gemtek and 220 Remington?

And I would NOT think that would be near as long as the supersonic round which is running at twice the velocity?

Either way, it seems the 9" 1/7 barrel is VERY handy for the reasons a PDW exists whether it's throwing a super or sub. I imagine the subs would be for indoor use with a can but it's less handy with a can for sure. I guess it would be great if you can have a well defined purpose before loading up. That would also be nice before choosing a particular weapon!

 

[Hairy Biker]

OK, I was wrong with the subs data. The 300 WTF/BLK/Whisper consumes the powder in a nine inch barrel at SUBSONIC with a 220 grain load. But for lighter bullets going SUPERSONIC there will be burn issues and subsonic with lighter bullets is outside the original design spec.

http://www.thetruthaboutguns.com/2014/11/foghorn/ask-foghorn-questions-300-aac-blackout-rifle/

For 300 AAC Blackout, that point is 9 inches of barrel. That number comes from multiple conversations with the people who designed the 300 AAC Blackout cartridge, did the initial testing, and who continue to design guns around that caliber.

 

[David4570]

OK, I was wrong with the subs data. The 300 WTF/BLK/Whisper consumes the powder in a nine inch barrel at SUBSONIC with a 220 grain load. But for lighter bullets going SUPERSONIC there will be burn issues and subsonic with lighter bullets is outside the original design spec.

http://www.thetruthaboutguns.com/2014/11/foghorn/ask-foghorn-questions-300-aac-blackout-rifle/

For 300 AAC Blackout, that point is 9 inches of barrel. That number comes from multiple conversations with the people who designed the 300 AAC Blackout cartridge, did the initial testing, and who continue to design guns around that caliber.

That's what I had heard when I bought the AAC 9" upper, but at my age and with my experience, I discount most everything I hear. But I feel this site has tremendous cred!

 

[Ayoung1]

I'd try to stay with 1:7 if you will primarily shoot subs. Mine is a 1:7 Seekins 8" barrel, compared to a friends 1:8 mine does much better beyond 50 yards. We shot some 220 smk to 100 and 200 yards, mine was holding 1.25 moa at 100 yards and his wouldn't even hold 4 moa. At 200 he couldn't keep them on a 12"x12" target where mine was holding 3-4 moa and bullets were stabilized. We used handloads mainly but got the same results with off the shelf ammo.