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  • Comparing stability and ballistic coefficient

    Posted by inconspicuousguy2 on June 24, 2022 at 2:22 am

    I was playing around with the Berger Twist Rate calculator, and came upon something interesting. Berger recommends using a bullet with an SG of 1.5 or greater for maximum stability. However, bullets between an SG of 1 and 1.5 are classified as “marginally stable.” Berger says you may get good groups, but you won’t be able to take full advantage of the bullet. Here’s where it gets interesting. Take this scenario for instance, using a 1 in 11 twist .30-06.

    According to the Hornady reloading manual, a 168 grain ELD Match bullet has a G7 BC of 0.263. At a maximum velocity of 2800 fps using the max charge for H4350, it has an SG = 1.81.

    That same manual states that a 200 grain ELD-X has a G7 BC of 0.301. At a maximum velocity of 2500 fps using the max charge for H4350, it has an SG = 1.22, and an adjusted G7 BC of 0.276.

    Now, according to the calculator, the 168 grain bullet has greater stability, and the 200 grain bullet, while having lesser stability, is not classified as “unstable” (SG = 1 or less), and it has the greater G7 BC. I realize BC isn’t the end-all, be-all, but I’m using it as a reference point.

    There’s many different combinations to compare. Is there a point where having less stability, but not being unstable, is outweighed by having a higher BC? Or is it always better to go with the higher SG, at the expense of BC and heavier weight bullets? Does it depend on the range and cartridge you want to shoot, and if it does, is there a cut-off point where you should choose one over the other?

    inconspicuousguy2 replied 1 week, 4 days ago 2 Members · 3 Replies
  • 3 Replies
  • Teddyturntup

    Guest
    June 24, 2022 at 2:22 am

    Oof.

    It is not always better to go with a higher sg, you just need *enough* stability to do what *you want to do* and then you worry about what bullet said barrel likes, and what bc you want/need/can get to minimize conditions

    Now what sg you need is Pandora’s box, Tooley cleaned up shooting 30 Cals in a 14 twist in long range Br. People now would find that ludicrous but they grouped and that’s what’s needed in that discipline. There is some talk that increased stability to a certain point helps with energy dump on game and with keeping bullets going in a line on game, so if that’s something you need a bullet to do it may be worth paying attention to.

    Tldr imo no, sg is not what you need to be chasing. You need twist enough to stabilize and after that chase other shit. Namely, your shooting ability and your wind reading ability. Shoot for 1.5 or above and get on with it

  • HollywoodSX

    Guest
    June 24, 2022 at 2:22 am

    Also keep in mind that stability issues can compound rapidly as you approach transonic and subsonic distances. If you’re staying safely inside TS range, then you can get away with the lower stability, but it can be a serious problem if you’re pushing past TS range.

    Inverted Example – the 168 SMK is infamous for not staying stable through TS range due to the boat tail angle, but if you spin them hard enough (8-9″ twist, IIRC) they will remain stable through TS and track consistently. The problem there is how few 308 rifles have the needed twist rate.

  • ifeelbetterlurking

    Guest
    June 24, 2022 at 2:22 am

    I first started looking at stability after seeing comments left by a Hammer Bullets-sponsored account (wish I could remember the name) regarding open tip hunting bullets. IIRC, they say that for open tip hunting bullets, like Hammers themselves or Bergers, you need a higher stability to ensure the open tip initiates terminal ballistics properly. Marginally stable just won’t work when trying to fill the freezer.

    Edit to add this is just more food for thought.

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