Deciding On Twist Rate - 6.5 CM

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otisrush

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This is kind of a follow-on question to my Throat Erosion thread from a few days ago. Given the lead times I'm seeing for certain barrels I'm strongly considering ordering a new barrel now so I can have it on hand when the current (.243) barrel throws in the towel.

I'm really leaning heavily towards rebarreling in 6.5 Creedmoor. The gun will continue to be used in the same way I've used it in .243: Steel silhouettes out to 500m; plus "plinking" (i.e. dabbling....not competing) shooting paper at 1,000 yds.

Using Berger's bullet stability calculator it looks like both a 1:8 and a 1:7 twist would work well, but the 1:7 produces higher stability scores than the 1:8.

Is stability something where more is better - always? Or is it more a scenario where once it gets over a certain amount (e.g. above Marginally Stable using Berger's terms) more doesn't matter?

From a pure numbers standpoint 1:7 looks like the right answer. Are there reasons I wouldn't want to do that?

Thanks.

OR
 
I believe the 6.5 CM was developed using the 1:8 twist barrel, 120-140gr bullet. If you planing on shooting the heavier super long ELD bullets go with the faster twist. Quite a few of the new bullets I've looked at call for a 1:7.5 twist or faster. Over twisting a bullet normally does not hurt anything, but may scrub off some velocity on the light bullets.
 
I am running a 1:8 twist and shoot 140gr Hornady ELD-M with no issues at all. My most accurate gun and accurate round out of all of my rifles and all calibers that I shoot. Behind it is 41.5gr of H4350 with a Fed-M large primer.
 
I recommend reading Bryan Litz's (currently Berger's chief ballistician/engineer, top-level ELD shooter, former rocket/missile scientist) various books on modern long-range shooting. He says that he found evidence in testing that actual BC's were better for "super stabilized" bullets. Basically, more spin is better for long-range stuff in terms of maintaining BC - and, therefore, velocity - over long ranges.

On the other hand, he also says, higher twist rates will add to any inaccuracy caused by non-concentric bullets. If a lead core is even slightly out-of-center in the projectile, it will introduce an asymmetric centrifugal force. The faster the twist, the bigger this force gets.

If what you are planning to do is mostly shooting at 500-1,000 meters, that would suggest the faster twist will be better. But it will less forgiving of cheap bullets.
 
depends on what bullet you decide to run. my personal preference is I think the 6.5 creed runs the 120-130gr bullets the best to get a high bc and plenty of velocity. example: 130gr nosler rdf. but, some people run the 140+ weight bullets just fine at normally a slower velocity. so a 120-143gr is fine with a 1-8", and anything longer/heavier would benefit from 1-7.5" or faster twist.
 
The 140’s only give up about 100fps to the 130’s, but they pick up 0.05 G7 BC or more The 130 is a .266, with the 140 Hybrid at .311 and the 140 ELD at .326. For the Hybrid, that’s 1/4mil less wind at 500, and only .1mil more drop. Only 1/2 of .1mil more drop for the ELD - and again, 2 clicks less wind (10mph standard).

500 is right at the cusp where velocity can’t outrun aerodynamics, so a lot of things can work.

Like many guys, I’ve found the 130 RDF to be incredibly picky for jump (140 moreso), which I wouldn’t trust for a competition where my throat erosion will increase my jump over a season. The ELD’s are incredibly forgiving and shoot very well. The Hybrids are also forgiving and shoot incredibly well. I can’t imagine an objective reason not to run one of these two bullets in a 6.5 for competition type use.

You do pick up some BC spinning them a little harder. I picked up about 0.01 G7 in the 6mm 105 Hybrid moving from a 1:8” to a 1:7” this fall.
 
Thanks all. I think I'm going to stick with 1:8. I got input from a friend who competes (very well) at the national level and he talked of various issues with over-spinning a bullet....including self-destruction (they can rip apart).

I'll check out Litz's stuff.

Thanks!
 
taliv said:
try a 135g JLK :)
Click to expand...

I’ve heard they really are more easy to get now, and it would be great if they were. Never been disappointed by any of their bullets, only in their supply inconsistency. Same reason I haven’t been shooting the 115 Dtac - although it seems his supply has leveled out of late too.
 
You should really consider a faster twist. A few days ago I saw a fellow shooting the 143 gr Hornady ELD-X Precision Hunter ammo out of a Thompson Center Compass with 1:8 twist. Every shot keyholed at 100 yards.
 
You should really consider a faster twist. A few days ago I saw a fellow shooting the 143 gr Hornady ELD-X Precision Hunter ammo out of a Thompson Center Compass with 1:8 twist. Every shot keyholed at 100 yards.
Click to expand...

Something else is wrong, it ain't the twist. I've been shooting those bullets for over a year now in 2 different rifles with 1:8 twists with excellent accuracy.
 
jmr40 said:
Something else is wrong, it ain't the twist. I've been shooting those bullets for over a year now in 2 different rifles with 1:8 twists with excellent accuracy.
Click to expand...
What else could be wrong? I'm not being sarcastic. I'm genuinely curious. I don't think there is anything the shooter could do that would cause the bullet to impact the target sideways. The bullet must be tumbling. So what are the possible causes, other than the bullet isn't spinning fast enough?
 
I had a 7 twist 223 barrel once that wouldnt stabilize heavy bullets. Took me a while to figure out the problem. I thought it might be demons. Or global warming. Turned out the Barrel was a 9 twist. Sometimes it pays to measure stuff
 
I agree! Measure stuff!

My ‘07 Rem M7 in .260 is supposed to have a 1/10” or even 1/9”.
By actually measuring, it’s close to a 1/11”.
Shoots “bug holes’ with a 120gr bullet. Good with Speer 140’s and discontinued Sierra 160 over RL22. (Flat base bullets-short!)

However, it shoots “patterns with a 129gr SST (4-6”), and won’t hit a 100yd berm with a 140gr SST.
I call it my “Short FAT” .257 Roberts.
 
Thanks for the info. I'm surprised to learn that QC at reputable firearms makers is so poor that the twist rate could be off by up to 30%! Seems like kind of basic thing to get right.

What is the process for measuring twist?
 
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