Ideal twist for 215 Hybrid

[Alex Wheeler]

Bryan, since we get to ask you directly now If you were to build a rifle around the 215 hybrid what twist you you use? Playing with your twist calculator, 9.5" looks about right?

 

[Bryan Litz Ballistics]

Alex,

Interesting question. As head coach of the Michigan FTR team, I've got a lot of insight to share on that subject, since most of our team (~15 shooters) have been shooting 215 Hybrids for the past 3 years from 9 and 10 twist 308 Win barrels.

In the previous 2 years, we shot 1:9" twist barrels, because there is a small but measurable improvement in BC for the 9 twist compared to the 1:10".

Although the 1:9"s have shot many 197's, 198's, and 199's, the only 200's shot by our team at 1000 yards (that I'm aware of) have been with 1:10" twist barrels.

Apparently, the slight edge in BC for the 1:9" twist is not outweighed by the inherent precision of the slower twist, even at 1000 yards.

I'm sure a 1:9.5" would work, but it's splitting hairs since we see such a small difference between 1:9" and 1:10".

Of course if your shooting the 215 Hybrids out of a larger case at higher speeds, the 1:10" may provide full stability and BC.

-Bryan

 

[Alex Wheeler]

Thanks Bryan, have you seen the velocity fall off much with the 9 over the 10 twist?

 

[Jay Christopherson]

That's an interesting response. Is that phenomenon something you've seen in other bullets/caliber combinations as well? Where a slight slower twist, although producing slightly lower B.C., yields better results on paper?

Is this going to be one of those "you'll have to test for your application" type deals or is there some sort of rule of thumb or something for predicting?

 

[Bryan Litz Ballistics]

Alex,

No practical effect on velocity. This has been tested directly and published in 'Modern Advancements in Long Range Shooting'. For 175's in 308 barrels from 1:8" to 1:12", the average velocity lost per inch of twist rate increase is 1.3 fps.

Jay,
There is a known correlation between twist rate and precision. How much effect there is depends on the quality (balance) of the bullets. For bullets having very good balance, you'll see a weaker correlation between twist rate and groups. The connection I've seen with the 215's and 1:9" vs. 1:10" twist hasn't been carefully tested in a scientific way, we just had a few guys clean targets with 1:10's, and no-one with 1:9's, although season long averages don't really favor one or the other. In other words, the link between 1:9 vs. 1:10 and precision is not a strong link in this case with 215's.

The link between twist/stability and BC has been carefully tested, and the relationship is programmed in the Berger stability calculator. That calculator is based directly on live fire results. Not for each individual bullet, but many bullets were fired with various stability, and the collective average effect of stability on BC is what's coded into the calculator.

-Bryan

 

[cmillard]

Brian, very interesting results. I wonder how this would fare out at longer range? do you think the 9" twist would get the edge?

 

[broncman]

Bryan,
How fast is your 215 load?

 

[Bryan Litz Ballistics]

Cole,

I'm talking about results at 1000 yards. If you're asking about ranges longer than that, I suppose it's possible that the 9 twist could have an advantage if you were to shoot into the transonic zone.

broncoman,

I'm around 2550 fps with the 215's in a 30" barrel, 308 Win.

-Bryan

 

[gilmillan1]

Bryan,

What powder are you guys using with the 215s?

I am in the process of building an FTR rifle.

Thank you.

 

[Bryan Litz Ballistics]

Varget has worked well for us with all bullet weights from 155 thru 215 in .308 Win. Some have tried other stuff but always come back to Varget. It can be difficult to find, so start looking now

-Bryan

 

[paulT]

Bryan, For .308 FTR Would running the 215H in a shorter say 28inch heavy profile barrel with 1 in 9.5 twist aid accuracy and reduce Torque over the longer 32inch barrel due to less bullet-barrel Dwell time thus helping keep vertical dispersion down to min along with the odd high flyer that occasionally open up groups at 900-1000y experienced by some FTR shooters running 32inch + barrels with the 215H also have you tried running the 215H at higher velocities from 2650 and 2725 fps over your 2550 fps loads thanks.

 

[Bryan Litz Ballistics]

We've tried barrels from 26" to 30"; 1:9" and 1:10" for the 215's. All work; it does seem like the shorter barrels are less sensitive meaning they provide better groups across various different set up's, rear bags, etc. With the longer barrels you better not alter 1 ounce of pressure on the rifle anywhere for any shot or you'll be looking at a flier.

Initially we ran 1:9" twists to optimize BC with the low velocities we were getting from the shorter barrels. The team is now a mix of barrel lengths and twists as we're moving away from standardizing around the 215's and trying other bullets; 185's and 200's. Our win at Camp Butner was a testiment to the virtue of mostly X-ring vertical with the mid weight bullets, even if it comes with more overall wind deflection.

-Bryan

 

[paulT]

Thanks for Info Bryan, I notice you were running your 215H in 2550 fps range which is easily achievable with a slower powder like H4350 in comparison to varget which has a much faster burn rate did you find best accuracy with varget for the 215H at 1000y, I also shoot the 200H which is an outstanding LR bullet and with my barrel shoots slightly tighter groups and vertical than the excellent 210 bt lr when run at 2780 fps range.

 

[calgarycanada]

Thanks for Info Bryan, I notice you were running your 215H in 2550 fps range which is easily achievable with a slower powder like H4350 in comparison to varget which has a much faster burn rate did you find best accuracy with varget for the 215H at 1000y, I also shoot the 200H which is an outstanding LR bullet and with my barrel shoots slightly tighter groups and vertical than the excellent 210 bt lr when run at 2780 fps range.

Is it even possible to get 2780 Fps from 308+200s? With sane pressures I mean.....

 

[dcali]

Regarding pressures - I hit about 2580 with the 215's over a stout charge of Varget out of a 32" barrel. I would not recommend it. Non-palma brass started losing primers on firing #2. "Easily achievable" is not a phase I would use to describe these velocities. As always, milage may vary, but that's my experience so far.

I'm also concerned that the 32" barrel is hard to shoot. It has been very difficult to get set up, and I'm still not happy with the accuracy.

 

[Canadian bushman]

I can push the 215's to a good 2600-2640 with h4350 and a 30" tube.
Around 2660+ i start getting loose pockets and random ejector marks.

I maxed out somewhere around 2730 with a highly compressed charge of rl-17, i think 49+ grains. It loosened primer pockets very quickly and flattened them consistently as well as easily identifiable ejector marks. Recoil was also difficult to tolerate in a 16lb gun.

 

[broncman]

I shot one match Saturday with 215's and did very well. Even had to use my first two for score to get in the 10 ring as I only had 23 loaded and no 1000yard zero.

This load has the bullet up in the throat to avoid the doughnut area. My elevation ended up at just under 32 moa. Plugging this in puts me right at 2600 fps. No heavy bolt lift but after inspecting the brass after the match about 40% had some ejector mark signs. I will drip powder and see if accuracy remains. Too hot fory liking, but it was a very good score for me even considering I had to use two of my for scores to get on target.

 

[gilmillan1]

Bushman, what is your load with 4350 to achieve 2600 fps?

 

[Canadian bushman]

Cci 200, 48gns h4350, jammed .005.
Im using .220 freebore. Loads are slightly compressed.

With fl sized brass it runs about 2630-2640

With neck sized it can run 2660-2670

Velocities checked with magnetospeed.

 

[gilmillan1]

What do you think would be a safe powder range using h4350?