20 Br with 55g Berger, what twist

[NZVarminter]

I've got a 20Br which has a 1:10 twist PacNor barrel. Initially I was shooting the 39 Sierra's, but I've now gone to the 55 berger and they seem to shoot ok at around 3650-3700fps.

I've put about 1200 rounds down this barrel now, so am planning for another barrel to replace once its shot out that will be dedicated for the 55g bergers.

I see that the Berger site says the 55's need a 1:8 twist, and have read a few guys are happy with 1:9, but not many shooting them with a 1:10 twist.

What twist is best for the 55's?

As a side note, I have looked at the Berger Twist calculator and ran it for the 39/40g at 3950 (18 deg C, 500m, 1:10 twist) and it gave surprising results:

Berger 40 has SG= 1.72
Sierra 39 has SG = 1.36 which is marginally stable
VMax 40 has SG = 1.32

I wasnt expecting hte Berger to have such a different stability factor compared to the BK and VM

These predictions dont seem right when most rifles chambered in 204 Rugers have 1:12 twists and shoot the 39/40 very accurately.

Cheers

Grant

 

[Hotrod6.5]

1:9 works for me. My wall hanger is .615 at 400yds for five shots. Shooting Reloader15 at 3626 with HBN coated Berger 55s. With a 100 yard zero I dial just 7.0moa for 600yds. I do think the 9 twist is marginal and my next barrel for this rifle is a Mullerworks 1:8 twist. This is a great little cartridge but tough on throats.
Rod B

 

[NZVarminter]

I'm thinking I might go for a 1:8.5

So how tough on the throat? how many rounds you get from yours?

 

[Hotrod6.5]

I got the rifle here on the forum and re-chambered it so not sure how many rounds were through the tube but i have about 500 since re cutting the chamber. Looking at the throat i can already see a little heat checking but rifling looks good. I dont want to guess as to life as i dont have good numbers for this barrel. I will shoot the 55 Berger's till accuracy goes away then re-barrel. The Mullerworks 1:8 looks as good as any i have ever seen and i expect it to shoot the long bullets great. I will keep accurate records on this one so will have some meaningful input for the 20BR folks.

 

[R.Morehouse]

Check out woodchuckden.com and give them a call about 20cal questions. Good site for related products as well.

 

[barefooter56]

NZVarminter,
Go with the 1-8' twist rate for the Berger 55 Long Range BT varmint.

 

[NZVarminter]

seen a few posts saying 1:8 twist causes blowups over 3600

1:9 seems best compromise

 

[barefooter56]

NZvarminter,
That can be an issue. 5R rifling and backing off the charge a little may help reduce the friction/heat that causes bullet failure. Varmint bullets do have a thin jacket. This is what makes them so explosive on varmints. The barrel seems to be the issue especially if its long (29-32 inches). But having said that. There are barrels out there that will shoot them all day with no issues. Luck of the draw I guess.

 

[Wirelessguy2005]

Barefooter56

I see you mentioned 5R rifling above, what are your thoughts on a 3 groove 9 twist barrel for this application? I ask because i am getting ready to build one similar.

 

[barefooter56]

Wirelessguy2005,
The 3 groove would help reduce the friction/heat issue also. The 1-9" twist rate? Go to our website and run the TWIST RATE CALCULATOR under the BALLISTICS tab. Don't forget to plug in your ALTITUDE. And see what your stability factor is and go from there. If you have any more questions please feel free to contact us here or at techsupport@bergerbullets.com.