6XC Twist Rate on new build

[effendude]

I'm rebarreling one of my F-class rifles to 6XC. I have two 6XC rifles with 26" 1-8" Kriegers and get awesome accuracy with the Sierra 107's. I have 1500 of the original Tubbs 115gr bullets and would like to try the Berger 115 VLD also. New barrel willl be a Broughton 5C finishing about 30" long. Should I go faster than 1-8" or will the increased velocity from barrel length offeset the slower twist for the heavy bullets? Thanks for your help.
Scott

 

[Yellow11]

The answer is in the maths... apply Don Miller's formula to get the ideal twist rate.

From memory the DTAC 115's need a 7.5 twist and the Bergers need a 7 twist, as stated on the manufacturer's box!

 

[spclark]

If you want to use the Berger 115 I'd suggest 1:7.5 or faster. (I think there's a warning on the packages I have for 1:7 or faster!) They don't so well at all at anything approaching 1:8.

I went with a 1:7.8 for a 1000-yd rifle & after two years of working with the DTAC 115 finally went with Plan B, the Berger 105VLD.

Maybe it's the twist, maybe it's just me but the 115's I've tried don't appear to be as easy to work with or as accurate & consistent.

I don't think velocity gained from a longer barrel - or more propellant - ever offsets the disadvantage a slower-than-necessary twist rate adds to the mix. Depending on the particular bullet and air density when they're fired, all have a minimum twist to produce stability. If you're slower than that they just flat don't work.

Where trouble lies is with faster twist and higher speeds - usually with lighter bullets than these - which tends to spin 'em so fast some may not make it to the target.... With a 6mm I think 1:7 may be the practical limit as anything faster may tend to really spike pressures as the bullets get up to speed.

If you stick to 600 yards & under the 105VLD is hard to beat. 115's aren't needed for 1000 either. Now this is just my opinion, so take it for what you paid me for it.

 

[Laurie]

Dr Geoff Kolbe, proprietor of Border Barrels in Scotland has a more sophisticated program than the Miller formula, but it needs more measurements.

Using data from Bryan Litz's measurements of the Berger 115 VLD and Sierra 115 DTAC from his book 'Applied Ballistics for Long Range Shooting', the Kolbe formula gives the following for 3,000 fps and 68 degrees F air temperature:

Berger 115: optimal twist 1-7"; Sg for 1-8" twist: c. 1.15
DTAC 115: .................. 1-7.4"; ....................: c. 1.30

for comparison
Sierra 107 MK: ............. 1-7.8; .....................: c. 1.44

Dr Kolbe's ideal Sg is 1.5 (compared to 1.4 in Miller) and that what this program's ideal twist rate results are based on attaining.

A 1.15 Sg is in my opinion really marginal, so you'd definitely want a faster twist barrel for the 115gn Berger, at least 1-7.5". I'd live with 1-8" and an Sg of around 1.3 with the DTAC, at least here in the UK where it doesn't get as cold as in much of North America.

In fact, I'll either have to, or eat my words, or have 1,000 DTACs for sale, as I just picked up my new 6XC 1,000yd BR Light Gun today from the gunsmith and it has a 1-8" twist Bartlein 5R barrel on it. I intend to use the Sierra 107 and Berger 105 as my primary bullets but bought the collection of DTACs too in case they turned out to be superior.

Actually, I do know they work at least reasonably well in this twist rate as top UK long-range shooter John Carmichael successfully used the DTAC in 6BR from a 1-8" twist barrel in 1,000yd F-Class matches a couple of years ago.


The link to the Border Barrels' facility is:

http://www.border-barrels.com/barrel_twist.htm


Laurie,
York, England

 

[effendude]

Thanks guys, I know the 115 DTAC bullets fly well in my OTC rifles with the 1-8" twist. But you guys know the OTC targets are not F-class.... As far as the Berger 115's, I'm just keeping my options open. I'll use whatever shoots the best as the competition here in Minnesota and all over is intense. I think I'll go with the 1-7.3" unless more responses direct me otherwise. Good luck with your new rifle Laurie!
Scott

 

[GermanS1]

Scott, I shoot the 115 Berger from a 1:7.5" twist Krieger with no stability problems but have seen them do very poorly with a 1:8" in a friend's rifle. I also have a 1:8" 6XC but limit that one to the 105-108 bullets.

 

[cmillard]

i have successfully shot the 115 bergers out of my 6BR .236 bore, 8" twist 5R krieger. so far, at 350 yards they are still stable--but i dont know what they are like further down range because that is the max distance at my range that i shoot at.

 

[flatlander]

I've shot both Berger & DTAC 115s extensively out of three 6x47Ls, and a 6XC. The most accurate of the 6x47s has a .237" bore 1:8 bbl., while the other two are .236"/1:7.5s. The 6XC is a .237"/1:7.8, and handles both the 115s fine out to 1000. While I've seen some nice 600yd. groups with both 115s, none of my bbls. groups as consistently with the Bergers, while the DTACs are very consistent.