6BR Neck Tension

[duke308]

Riddle me this one;
6BR, 30.5 gr Varget, 107 SMK
Forster Bump die with .265 bushing a 1.219 vertical spread at 200 yards and with a .267 bushing I get .741 horizontal spread.
How does the neck tension change the result in the spread?

 

[Texas10]

I'll be following this intently. I use a 266 bushing in RCBS Gold Medal FL die. Have you tried a 266?

I"ve read here that once seating depth and charge are established, neck tension is the final tuning procedure.

Can't wait to read what others have to say.

 

[Eddie Harren]

fguffey sells "neck tensions" along with his "datums".

 

[spclark]

In some cartridges "less is more".

I've settled on using as little effective neck tension in the cartridges I shoot regularly (308 for Palma mostly, 6HAGAR but not much lately, a 7mm wildcat with a neck just 3mm long...) because I've learned this gives me the most consistent, predictable accuracy out to 1,000 yards which includes minimal vertical dispersion.

Some will suggest greater neck tension allows powder burn to be more efficient but the effects of less tension down range are what I'd rely on more than conjecture about what happens in the chamber & case once primer ignition gets things moving. I put it down to harmonics: less neck tension makes for a less violent bullet release at the moment it passes the crown. No proof of this opinion other than what I see down-range when my targets get pulled for marking/scoring... unless I'm on an electronic target system of course!

 

[Alex Wheeler]

Neck tension is very important and what shoots best will change dependent on the bullet and powder you shoot, even the lot of bullets can change it. You should never settle on any amount of tension. Test it every time.

 

[Richard Coody]

+1

I bought a whidden full length sizing die with the.expander ball kit. Use the .243 expander ball. Use a 21st century hydro press and my seating pressure runs about 30 or 35. Feels very light. Shoots bugholes.

 

[.22gongzilla]

Neck tension has a big effect on consistency when you're jamming into the lands, I found that out.

 

[lpreddick]

If you believe in Chris Long's "shock wave" theory you can understand how varying neck tension will affect a bullet's POI and a group's movement as neck tension vary . According to Long, a shock wave (vibration, barrel harmonics) moves down the barrel at propellant detonation, then returns to the breach, then back to muzzle, repeating till damped out... several times before bullet exit. Bullet arrival at muzzle when shock wave is a breech is the ideal. We affect bullet's arrival at muzzle by several means, neck tension being one. Mild (loose) tension requires a specific pressure to move bullet while a tighter tension will require more pressure to move bullet. Higher pressure results in greater speed of bullet, thus arriving at muzzle sooner than prior one. This bullet's arrival at muzzle in relation to location of shock wave affects it's POI as well as group size. Load 10 cases, same charge, same bullet, same seating depth, but .001 neck tension for 5 and .003 tension for 5. You might see two tight groups but different locations. You can see this difference even if neck sizing is the same but one bullet seated with more resistence (not annealed), therefore neck grip is greater and this case requires more pressure to cause release...my observations.

 

[tenring]

If you turn necks, tension will be more consistent. I only shoot tight necks and turn everything.

I recently tested various bushing sizes in two calibers, a factory .223 and custom 30BR - .330 neck. In both instances, with identical loads, tighter neck tension improved accuracy. But there is a point of diminishing return. In the 30BR, I went from .326 bushing down to .320, which is the one that shot best. A .319 went the other way (shot poorly). The .223 is a factory chamber, not tight neck. I lightly skimmed the necks, and went from .247 down to .240, which was most accurate.

 

[Eric Leonard]

I give up on the whys and just try and use what works best,,

 

[BoydAllen]

Test, and understand that your results are only valid for the powder that you used. What one powder likes, another may not.

 

[fgregorio]

Funny I was testing that today. Seating with a hydroseater: I shot 55 psi- not accurate/ flatter trajectory. My sweet spot is at 20 psi. I shoot the rounds in order from the heaviest seating pressure to the lightest seating pressure. If you use a hydroseater, you don't have to worry much about choosing bushings by trial-and-error. Just choose a bushing by testing seating pressure. To do minor adjustments just go:
Heavier pressure- anneal, size and wait three days before seating.
Medium pressure- anneal, size and seat on the same day.
Light pressure- anneal, wait three days then size and seat.
Makes sense?

 

[BoydAllen]

That information is pure gold. Thanks.

 

[spclark]

Light pressure- anneal, wait three days then size and seat.

Yep, what I do too.

 

[Mulligan]

Funny I was testing that today. Seating with a hydroseater: I shot 55 psi- not accurate/ flatter trajectory. My sweet spot is at 20 psi. I shoot the rounds in order from the heaviest seating pressure to the lightest seating pressure. If you use a hydroseater, you don't have to worry much about choosing bushings by trial-and-error. Just choose a bushing by testing seating pressure. To do minor adjustments just go:
Heavier pressure- anneal, size and wait three days before seating.
Medium pressure- anneal, size and seat on the same day.
Light pressure- anneal, wait three days then size and seat.
Makes sense?

Wow!
What is the difference your seeing on paper with the same load and some of each prepared as stated above?
Very interested in this..........
CW

 

[foxguy]

Funny I was testing that today. Seating with a hydroseater: I shot 55 psi- not accurate/ flatter trajectory. My sweet spot is at 20 psi. I shoot the rounds in order from the heaviest seating pressure to the lightest seating pressure. If you use a hydroseater, you don't have to worry much about choosing bushings by trial-and-error. Just choose a bushing by testing seating pressure. To do minor adjustments just go:
Heavier pressure- anneal, size and wait three days before seating.
Medium pressure- anneal, size and seat on the same day.
Light pressure- anneal, wait three days then size and seat.
Makes sense?

Can you post what you consider to be the Seating Pressure Ranges in PSI
for the Heavy, Medium and Light Pressure ranges? Just curious...

Thanks

 

[fgregorio]

Sure.
Heavy: 50psi- 30 psi
Medium: 29psi- 20 psi
Light: 19psi- 10 psi
I reject below 10 psi and above 50 psi.
Tested last Saturday, the poi difference between 50 psi and 10 psi was 2 moa at 300m.
Barrel: .3075 Bartlein 5R slugged to 28 inches from a 35 inch blank.
Projectiles: Optimus 155 gr pointed with a Hoover system coated with HBN milligram batched
Cases: Winchester fully tricked out. 0.1 gr batched.
Powder: 46.9 gr Varget milligram weighed
Jump: 23 thou
Neck tension in stage 20psi-18 psi batched and shot in order from heaviest neck tension to lightest neck tension.
Primers: 210M milligram batched.
MV: 3180 fps SD 4 fps
Time of flight to target: 318 milliseconds to 300 metres SD 1.5 milliseconds

 

[dmoran]

Some input from me (based from 243/6mm experience): I am of the opinion bullet diameters (and/or bullet pressure bands) effect the amount of optimal tension needed. IME - larger diameter bullets (or bands) typically need more tension. And of course, neck preparation can have substantial effect.
Just my 2-Cents
Donovan