7.7 vs 8 twist?

[xswanted]

I'm looking at a 22cal blank for another 22br build.

bugholes has a Bartlein with a 7.7 twist.....whats the deal with that?

Is that fast enough for a 90?

Is it too fast for a 80SMK?

I have a 8 twist right now shooting the 80s.

Would this perform just fine with the 80s and 75s?

 

[swehrman]

It will be just fine with the 75's and 80's. 1-7.7" is one of the more popular AR-15 service rifle barrel twist rates.

-- Scott

 

[xswanted]

Sounds good thanks!

 

[Hosted]

I don't know much about the 22br velocity but I have a feeling you would be able to shoot 90's with a 7.7 twist if it was fairly long. The link below by jerryhm is some fantastic records showing different bullets and barrel lengths with his 223 with a 7.7 twist. He would have been right on the edge of making it work but obviously it did for him.

http://forum.accurateshooter.com/index.php?topic=3634784.msg35817715#msg35817715

 

[xswanted]

I don't know much about the 22br velocity but I have a feeling you would be able to shoot 90's with a 7.7 twist if it was fairly long. The link below by jerryhm is some fantastic records showing different bullets and barrel lengths with his 223 with a 7.7 twist. He would have been right on the edge of making it work but obviously it did for him.

http://forum.accurateshooter.com/index.php?topic=3634784.msg35817715#msg35817715

Sweet thanks.

It'd be cool if I could, but not a needed thing by any means. Thanks for the link!

 

[TominSWNY]

7.7 twist worked with the old Berger 90 gr bullets in a 22BR, but don't work with the new 90 gr Berger's. Had excellent groups with the old bullets and patterns with the new .

 

[Laurie]

7.7 twist worked with the old Berger 90 gr bullets in a 22BR, but don't work with the new 90 gr Berger's. Had excellent groups with the old bullets and patterns with the new .

That's because the twist rate always was marginal for this length of .22 cal bullet. A very small change in bullet production lengths or in ambient conditions can see it work or not work. 7.5 is the minimum rate for 90gn VLDs in the calibre and 7 is better still as an insurance.

 

[Hosted]

I haven't tried them so I'm just going off of numbers but the JLK 90VLD is slightly shorter supposedly making it more stable than both the Berger VLD and the discontinued Match bullet. Just another option if you ever go down that road. Once I get my 223 together I'm hoping to see how they compare.

 

[TominSWNY]

JLK 90VLD from the web site requires a 6.5 twist.

 

[Laurie]

JLK lists their 90 as 1.225 inches OAL. Put that into the Millers Twist Rule spreadsheet with a 1-7 twist and you get a 1.46 stability coefficient value at 2,850 fps (quite achievable in .223 Rem with a prone match rifle) under standard ballistic conditions - an optimal value.

Change the atmospherics to 32-deg F and 30.5 inches mercury, ie a high pressure winter day and it's still 1.36 which is plenty stabilised.

The 1-6.5 inch rate was originally quoted by Sierra for its 90 in short barrel 223 AR-15s in Service Rifle mode. It's regarded as far too fast a twist for most applications. It, and the 90 SMK's construction, was one reason for a high incidence of bullet in-flight blow-ups which set the use of 90s back maybe 10 years until the issues were sussed out.

 

[Hosted]

Just FYI, I just talked to Swampworks about getting a sample box of JLK 90VLDs to load up a dummy round and ship off for a barrel and he said they are discontinuing the 90s right now because J4 is no longer producing those jackets. It may not be a bid deal, but it sure doesn't leave many choices in the 90gr family. :-\

 

[LCazador]

Been there done that with 22 BR! Absolutely 7 twist min. for 90's, and yes it will be fine for 80's not sure about 75's never did those. That 7.7 will not do 90's very well but then there are some people that like the shotgun effect, in that case get a cylinder choke ;D!

 

[xswanted]

Thanks everyone.

I'm not really concerned with shooting 90s, it would just be an added bonus if it worked.

I have a 22BR right now with an 8 twist and its unreal good shooting with the 80s, especially past 200 yards. I just wanted to make sure a 7.7 twist shooting the same bullets isn't going to be some kind of tuning nightmare.

 

[Beau]

The 7.7 will do better with 80 AMAX and 80 VLDs (at least at .223 velocities).