Base to ogive vs bearing length



  • Which is the best way to sort boat tail bullets to tighten the velocity spread or make more consitent ammo for long range?
    I am currently sorting by bearing length and noticed that depending on what lot I use I get different cartridge base to ogive length consistently.



  • @toni said in Base to ogive vs bearing length:

    Which is the best way to sort boat tail bullets to tighten the velocity spread or make more consitent ammo for long range?
    I am currently sorting by bearing length and noticed that depending on what lot I use I get different cartridge base to ogive length consistently.

    I did an experiment a few months ago on how the difference in bearing length effects velocity and POI for my .308. I had a spread of .033 on a box of 500 168SMK's and so I took at the pile from each end of measurements to compare and see what kind of difference I might get. I also measured these bullet's base to ogive and their weights for this experiment. The measurements I took from base to ogive didn't show anywhere near that much difference and often no correlation on individual bullets. While there was some corresponding difference in weights, that too didn't show as much in differences.

    What this difference in bearing length did when seating the bullet was there was a significant difference in seating depth though I make no changes to the seating die. And apparently, this different in seating depth is what seems to have affected the MV's substantially between these two groups of bullets (I fired 20 rounds from each of the two groups of bullets), which as you might expect, the POI were affected too.

    Here's some numbers:

    168 SMK's loaded with 41.0gr Varget into Lapua brass with Federal primers and set to .010 of my lands. Used a Magnetospeed V3 to measure MV

    Shortest bullets group measured .466 - .467 in.
    Average MV - 2438 fps
    SD - 13.1
    ES - 31

    Longest group measured .499 - .500 in.
    Average MV - 2474 fps
    SD - 12.5
    ES - 42

    That's a difference in in average MV of 36 fps.

    alt text



  • @straightshooter1 said in Base to ogive vs bearing length:

    @toni said in Base to ogive vs bearing length:

    Which is the best way to sort boat tail bullets to tighten the velocity spread or make more consitent ammo for long range?
    I am currently sorting by bearing length and noticed that depending on what lot I use I get different cartridge base to ogive length consistently.

    I did an experiment a few months ago on how the difference in bearing length effects velocity and POI for my .308. I had a spread of .033 on a box of 500 168SMK's and so I took at the pile from each end of measurements to compare and see what kind of difference I might get. I also measured these bullet's base to ogive and their weights for this experiment. The measurements I took from base to ogive didn't show anywhere near that much difference and often no correlation on individual bullets. While there was some corresponding difference in weights, that too didn't show as much in differences.

    What this difference in bearing length did when seating the bullet was there was a significant difference in seating depth though I make no changes to the seating die. And apparently, this different in seating depth is what seems to have affected the MV's substantially between these two groups of bullets (I fired 20 rounds from each of the two groups of bullets), which as you might expect, the POI were affected too.

    Here's some numbers:

    168 SMK's loaded with 41.0gr Varget into Lapua brass with Federal primers and set to .010 of my lands. Used a Magnetospeed V3 to measure MV

    Shortest bullets group measured .466 - .467 in.
    Average MV - 2438 fps
    SD - 13.1
    ES - 31

    Longest group measured .499 - .500 in.
    Average MV - 2474 fps
    SD - 12.5
    ES - 42

    That's a difference in in average MV of 36 fps.

    alt text

    Interesting that such a slight change in seating depth could make a 36 fps difference.



  • @martino1 said in Base to ogive vs bearing length:

    Interesting that such a slight change in seating depth could make a 36 fps difference.

    Though I noted what I would say was a "significant" difference in seating depth (about .004 or so, maybe more, as I try to recall), I failed to record the actual numbers for that. And I would not attribute all of that velocity difference to seating depth as their's the friction coefficient difference for the bearing surfaces and statistical difference in loads.

    Due to this little experiment, I'm convinced that sorting by bearing surface length is better than sorting by measuring base to ogive. . . . at least until I can see any data to the contrary. And since this experiment I've been loading accordingly and find my COTB has been very consistent with variations around +/- .0005 using my Forster Ultra Micrometer Seating Die.

    After finally figuring out my balance scale and getting consistent seating depths I'm getting the kind of results I was aiming for (single digit SD's and ES of less than 30). Now if I could only be a consistent shooter. ;-)



  • @straightshooter1 said in Base to ogive vs bearing length:

    @martino1 said in Base to ogive vs bearing length:

    Interesting that such a slight change in seating depth could make a 36 fps difference.

    Though I noted what I would say was a "significant" difference in seating depth (about .004 or so, maybe more, as I try to recall), I failed to record the actual numbers for that. And I would not attribute all of that velocity difference to seating depth as their's the friction coefficient difference for the bearing surfaces and statistical difference in loads.

    Due to this little experiment, I'm convinced that sorting by bearing surface length is better than sorting by measuring base to ogive. . . . at least until I can see any data to the contrary. And since this experiment I've been loading accordingly and find my COTB has been very consistent with variations around +/- .0005 using my Forster Ultra Micrometer Seating Die.

    After finally figuring out my balance scale and getting consistent seating depths I'm getting the kind of results I was aiming for (single digit SD's and ES of less than 30). Now if I could only be a consistent shooter. ;-)

    I haven't loaded any of my 142 SMK 6.5s that I got to replace the hard to get Berger 140 hybrids. I think I'll sort the 1000 I got to try to get the SD under 10. I think I was at 10.6 or so with the Bergers and no sorting. Need to do a Load Dev as soon as I get the time. Still swamped with tax season work on extension.

    I did get a second Hornady measuring base and 6.5 insert to measure the contact surface the same way you do.



  • @straightshooter1

    Wow .033 spread of bearing length is a lot, I sorted a few boxes of bergers vld and got a bearing length spread of .004

    I measure the same way too with two hornady inserts.



  • @toni

    Yeah . . . "WOW" was my thought too, at the time. That much spread prompted me to do that little experiment to see just what kind of different it makes. Since then I've gotten a new box of 500 and after measuring all of them, there was only a .007 spread.


 

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