6.5 mm Optimum Barrel Twist?

[rcw3]

For years I have used an 8.5" twist with my 6.5 x 284 barrels shooting 140-142 gr bullets, and not knowing any different, I have been happy with that. I regularly see shooters using an 8" twist. I know a 9" twist is right on the edge (or even over the edge stability wise) with some of those bullets, so now I am wondering if an 8" twist is actually a better twist (stability wise) for the 140-142 gr bullets. Maybe we're shaving hairs here, but I would be interested in any commentary by shooters on this. Obviously there are those who feel an 8" twist is advantageous over an 8.5" twist, so the question is: What is the factor that prompts you to buy an 8" twist vs an 8.5" twist?

Robert Whitley

 

[1shot]

Hello Robert,
I have an 8 and an 8.5” twist both by the same mfg. They are the same length and chambered with the same 6.5x284 reamer. The 8.5 will always give me more velocity with less pressure than the 8 twist. More importantly as we both know, at the velocities that I run I’ve never had a “failure to arrive” with the 8.5 twist bbl. I can’t say the same thing about the 8 twist. That said, if I slow them down, or use a thick-jacketed bullet, no problems, but with thin jackets, if the conditions are right, I will sometimes lose one late in the string. I’m sticking with 8.5 twists unless someone can prove to me there is a better mousetrap out there.
I hope this helps,
Lloyd

 

[Obtunded]

I deal with lots of customers that have been very successfully using 9" twists in 260AI and 6.5-284 with 140 Bergers and 142 Sierras. I do not see this as being the least bit controversial. As long as your twist is remotely accurate, it will stabilize heavies

 

[Rust]

Greetings Robert,

I don't really understand where some of the bad rap concerning 1-9 twist barrels in 6.5 has come from. It's all I've used from the first barrel and I've never had the least bit of trouble from a chilly winter day to a smoking hot Alabama summer day. Preferred bullet a 139 Scenar at 2950 fps. The barrels have been 1-9, not close to 1-9, but 1-9.

Spent a good portion of the first barrels life sorting through powder and primer combinations looking to optimize the combination of velocity with low ES/SD numbers. It worked, I've chrono'd in low single digits. Not exactly the most common combination but pretty close to it. I'll go so far as to say that uniform neck tension was the last part of that puzzle.

Since the first barrel, I spec'd a new reamer (.297 neck which is tight for a no turn neck) which happens to work well with my Redding dies and doesn't over work the brass, and altered the freebore just a bit from the original reamer. Just that last bit of fine tuning based on that first barrel and the very few things I thought could be improved at all.

Bottom line is what works, works.

Sometimes I think that requiring more twist in some barrels than others, given the same bullet and same velocity is an indication that either the twist isn't exactly as stated or that the twist may not be uniform the entire length of the barrel, or both. Even gain twist barrels need to be very uniform in tiwst once they get past the section with gain.

 

[500 meter ram]

I've been using 8 twist on my silhouette rifles with 141gr and 142 gr bullets because thats what "everyone said" had to be used. Thought about using a Krieger 8.5 twist but was afraid of spending the money and then finding out it was a mistake. I only shoot the heavy bullets at around 2650 fps or so. Any comments? Sierra said that 8.5 twist wouldn't stabilize their 142's.

 

[1shot]

500 Meter Ram,
An 8.5 will work just fine as long as you keep the velocity up. At the speeds you stated, I would have to agree that stability could be a problem, but I don't have any hard data to back that up, as I run mine just short of 3,000 fps.
I hope this helps,
Lloyd

 

[500 meter ram]

Thanks Lloyd. I suspected as much since Sierra kept asking me what cartridge I was using. I couldn't figure why they just didn't want to know the speed. I'll save my money for another project.

Chuck

 

[Rust]

There is a minimum RPM needed to stabilize the bullet, and although the twist of the rifle contributes the majority of the RPM, velocity contributes a fair portion as well. As stated earlier I'm using the 139 Scenars which are pretty much equal to the 142 Sierras at 2950 fps with a 1-9 barrel.

Noww I don't know how it would have worked at longer ranges, but at 100 yards during initial load developement they seemed stable all the way down to 2750 or so.

But a big "your milage may vary" as there are probably a number of things which could contribute one way or the other. For instance, the 1-9 barrel needs a very straight bore with a very straight chamber and the bullets seated perfectly straight. Should the bullet exit the muzzle less than nearly perfectly, it could upset the apple cart and the bullet.

;D ;D turns out my smith is very, very good ;D ;D

 

[robbin]

Hi Robert,
It's simple...Bryan Litz, the Cheif Ballistician for Berger Bullets says you need 1:8 twist for the 6.5 140. Good enough for me.

By the way, he has an awesome book out called Applied Ballistics for long range shooting. Written for shooters, not math majors.

Good luck,
Robbin

 

[rcw3]

Robbin

Here's the twist (pun intended) with your posting. Berger Bullets, only a very short time ago, listed the twists on the bullet boxes for the 140 gr BT and VLD bullets as 1:9" or faster, so I suspect there's a little margin built into any newer 1:8" recommendation. Berger, in making a twist recommendation, has to anticipate someone shooting them very slow at sea level in very cold weather, even at subsonic speeds, so I suspect the 1:8" is the closest full faster twist number taking that all into account.

Robert Whitley

 

[wwbrown]

I have built a bunch of 6.5's and have always run with a 1-8" twist rate. The lightest bullet I shoot is 130 g Berger VLDs. Bartlein says the most common twist that people order in the 6.5 is 1-8.5" twist.

I imagine either rate would work, I stick to 1-8 as this has always worked for me and I try to abide by "If it ain't broke don't fix it."

 

[AP2020]

1 in 8 on my Pac-Nor, 6.5WSSM