How much effect does neck tension have on volocity

Discussion in '6BR, 6BR Improved & Wildcats' started by South Prairie jim, Jul 5, 2018.

  1. South Prairie jim

    South Prairie jim

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    Gents
    I'm curious to the effect of neck tension on volocity.
    I'm assuming that inconsistent neck tension could create variations in FPS leading to inconsistent results on long range targets.
    What have others noticed and recorded in logs?
    Regards
    SPJ
     
  2. LA50SHOOTER

    LA50SHOOTER Gold $$ Contributor

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    My most recent experience with differences in neck tension was it caused the groups to "move" where they printed on paper at, & also the size of the group increased. - Recently I was doing some testing and I tried a 0.364 bushing for the exact same load, only difference was the neck bushing that was used. 0.364 printed to the right and also the 5 shot group was bigger (about 2.5 to 3 times bigger) than what the 0.363 bushing produced, which is normally what I use and my brass is neck-turned to 0.014 for a 0.369 neck chamber (.338 + .014 +.014 = .366) -I Was testing at 500 yards firing 5 shot groups in 338 Lapua Imp. using 300 gr. Berger Hybrids & a healthy charge of Reloder 33.
    - The load was tuned using a 0.363 bushing from the onset but I wanted to see what would happen if I changed (decreased) the neck tension. - I didn't see hardly any change in muzzle velocity at all & I was using the LabRadar to capture the velocities as I was shooting my groups. Both averaged within 10-15 fps of each other shot to shot. - Its obvious that the lesser neck tension was not in tuned for the gun under the same parameters.

    - Ron -
     
  3. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Not much. What it does do is release the bullet at a different point relative to the pressure curve which will effect muzzle exit timing. Thats how I see it anyhow.
     
  4. foxguy

    foxguy Silver $$ Contributor

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    Interesting, so it sounds like the 364 bushing's neck tension increased your Velocity 10-15 FPS and that took you out of your Node. I would of been curious to see if dropping your powder charge 1 or 2 tenths of a grain would of put you back into your Node??
     
  5. LA50SHOOTER

    LA50SHOOTER Gold $$ Contributor

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    I doubt 1 or 2 tenths would have had any effect at all, I'm talking 106.02 (measured to the kernel) grains of Alliant Reloder 33. - The load shot good at 105.02 grains as well but was about 30 fps slower. - I don't know where the "top end" is on the load with this powder, no "feel" on the bolt handle and the primer pockets are still good after a previous firing at the same powder charge. - I stopped at 106 because it satisfied a criteria that I'm trying to meet for the E2K match which was to push the 300 gr. Berger Hybrid to 2950 fps, the 106 gr charge yields 2963 with good accuracy and low ES & SD numbers (single digit).

    - Ron -
     
    Last edited: Jul 5, 2018
  6. South Prairie jim

    South Prairie jim

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    Would you see groups moving left or right of point of aim?
     
  7. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Sure you could. Barrel harmonics are not just in the vertical plane.
     
  8. shoot4fun

    shoot4fun Gold $$ Contributor

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    Neck tension has an effect on bullet seating depth too.
    Sort a few bullets by base to ogive and then seat them into cases. When you feel more seating pressure on a particular case you will invariably see the bullet not seated as deeply as the others that had the same seating pressure. It works inversely too. If you have one with much less seating pressure then the bullet will tend to be seated too deep compared to the rest.
    Alex hits the nail on the head though. Variance in seating pressure/neck tension will cause flyers.
     
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  9. South Prairie jim

    South Prairie jim

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    Oh boy!
    I seen flashes of brilliance as in 7/8 inch groups at 550 then the next group wild!
    Kinda pisses a guy off a bit however it's all part of the journey right..
    Thanks guys
    Jim
    Just a side note I had volocity swings yesterday from 2883-2902 that was a hot one!
    Gents after checking my notes the variations on that load were 2890-2902
     
    Last edited: Jul 6, 2018
  10. Rushty

    Rushty Gold $$ Contributor

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    I’ve also seen the same corolation between neck tension and bullet seating depth..tighter ones generally always seat longer and softer ones usually seat where I want them or a touch deeper..
     
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  11. DukeDuke

    DukeDuke

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    does neck tension matter if the bullet is jammed in the lands?
     
  12. BoydAllen

    BoydAllen Gold $$ Contributor

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    If you have light neck tension your bullet will not go as far into the rifling as it would if you have heavy tension. IMO the farther a bullet is into the rifling, the more pressure it takes to make it start moving when the cartridge is fired.
     
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  13. milanuk

    milanuk Team Savage Gold $$ Contributor

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    I'm not arguing the other parts... I've absolutely seen the seating depth vary with the seating force/pressure.

    But I'm not following where the 'sort a few bullets by base to ogive' comes into that equation...?
     
  14. South Prairie jim

    South Prairie jim

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    Rushty
    Would you readjust those rounds that seat longer or shorter
    Jim
     
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  15. shoot4fun

    shoot4fun Gold $$ Contributor

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    What I'm saying by sorting is you remove a variable for the "experiment". My feeling is that variance in BTO cause bullets to seat differently due to the shape differences. Isn't that really what you have when bullets measure differently? Maybe not using the correct wording here but I hope you see my point.
    As for sorting, in general, I sort only bullets that I am meplat trimming or jamming.
     
  16. Rushty

    Rushty Gold $$ Contributor

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    No I don’t readjust, generally I reload enough for a match to include foulers.. anything that seats hard or soft gets allocated to foulers..
     
  17. shoot4fun

    shoot4fun Gold $$ Contributor

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    Same here.
     
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  18. South Prairie jim

    South Prairie jim

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    That's really good feedback Ron
    I've noticed that my 6 mm br .243 +13+13 = .269 however a loaded round measuring.268 is the Datum I used when ordering a bushing in my case .266 -.002
    the interesting thing is when I double check with a .241 Mandrel I'm slightly under .002 bullet hold more like .0015 barely enough to hold seating depth, I've also noticed that my groups are very erratic lately leading to a new .265 bushing ( in the mail ) understanding that each rifle is different I'm hoping to clean things up a bit.
    I also checked my notes on the recent FPS 30.2 gr Varget put out 2890-2902 not great ES if that's the term but better than I remembered ' sadly I'm getting old.
    Thanks Gentleman
    Jim
     
  19. milanuk

    milanuk Team Savage Gold $$ Contributor

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    Hmmm... on the one hand, I'd say 'No', as the BTO measurement (in my mind) is looking at the bearing surface plus the boat tail. The seater stem is contacting a completely different area on the bullet nose cone.

    On the other hand, I haven't sorted by BTO in any serious capacity (until recently, as I'm doing a multi-sort - OAL and BTO) so I haven't actually *tested* it as such. Have you been able to establish a definite correlation between variation in BTO measurement and seating depth variation in your efforts?
     
  20. Martin52

    Martin52

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    This is the reason I sort using the Bob Green tool.
     
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