Can somebody explain ogive numbers to me, eg...7/10??

Discussion in 'Reloading Forum (All Calibers)' started by X-47B, Mar 26, 2020.

  1. X-47B

    X-47B X-III:XVI Gold $$ Contributor

    Feb 26, 2018
    Noticed a lot of custom bullets have measurements for their ogives. #\#. Trying to understand what it means and how it’s revelant.

    Thanks Guys
  2. Rdlningcltchdmpr

    Rdlningcltchdmpr Gold $$ Contributor

    Jan 3, 2019
    It is the pointyness. Some are longer pointy , some are more squat.- less pointy. It has an effect on the balance also in relation to twist rate.
    gunsandgunsmithing likes this.
  3. Laurie


    Oct 27, 2009
    There are two metrics: nose radius shown in terms of calibres. So, a 0.308 dia. bullet with a 7-cal radius conforms to the section of a circle with a 0.308 X 7 = 2.156 inches radius / 4.3" diameter. The higher the number, the longer the nose, the greater the 'pointiness' and the less drag. Elderly Sierra MK and similar designs are often around the low 7-9 radius values.

    The Rt/R metric tells you how abrupt or 'sharp' the shank to nose junction is and is obviously an effect of nose length and shape given that the shank is always parallel.

    Where the nose is well rounded and has a relatively low radius value, the nose section flows easily into the shank and has a high Rt/R value. The 168gn 0.308 SMK measures 0.85 and the later 175gn MK is 1.00. Such set-ups are called 'tangent ogive'. If you take a scale drawing of the bullet and a compass and extend the nose backward towards the bullet base, the line curves inside the bullet shank.

    Where bullets have very long gentle radius nose sections - as in the current very low drag Sierra MKs with nose radius values of around 28 calibres, you have a low Rt/R value and a secant ogive neck form. Do the drawing and compass exercise and the nose shape extended line stays outside of the bullet shank. A traditional Knox / Berger VLD secant ogive design has an Rt/R of 0.5, but some of the new super-nose SMKs are only a little above 0.3. The lower the value, the sharper the junction between the two bullet sections, but also the lower the drag that the bullet nose generates in flight.

    Low Rt/R value bullets ('aggressive' secant ogive / VLD forms) are as rule harder to 'tune' vis a vis COAL / jump; high value designs like older SMKs with values of 0.75 to 1.00 are jump-tolerant but usually generate more drag and therefore have lower BCs.

    The one bullet type where you don't get an Rt/R quote is the Berger Hybrid which incorporates both forms, a long secant nose that then 'transitions' into a more sharply radiused tangent form just above the nose to shank junction. In theory, it gives the low drag / high BC of a full VLD type but the jump / position tolerance of a traditional short nose tangent design.

    If you have a bit of spare cash and are looking to use the upcoming long periods of 'coronavirus house arrest' profitably, I'd strongly recommend purchase of two Bryan Litz / Applied Ballistics LLC books:

    Applied Ballistics for Long Range Shooting 3rd edition

    Ballistic Performance of Rifle Bullets 3rd edition

    The first is a complete external ballistics primer covering all topics on the subject including bullet design, type of BC and how they're measured / used etc and written for non technical people like me.

    The second is purely a listing of bullet models with drawings, drag, BCs, twist rates requiremets etc of some 950 or so bullet models of all types, calibres, and makes, everything except rimfire bullets and cast lead type models.

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