Bullet measurement request.

[SteveOak]

For some reason or another I was looking at the length of the 6mm Sierra 107gr MK and the 6.5mm Sierra 107gr MK (not the new TMK).

The JBM bullet length list says the 6.5mm Sierra MK is 1.275" and the 6mm Sierra 107gr MK is 1.227.

I have some of the 6mm 107s and measured them at 1.229 - 1.230", close enough but I am having a hard time understanding how a bullet of the same weight but larger diameter and significantly lower BC, .406 vs .547 @ 2,800 fps, could be longer.

If you have some 6.5mm Sierra 107gr MKs on hand and could measure a few and post your results here, I would very much appreciate it.

 

[6ShotsOr5?]

Steve, I don’t have either bullet to measure for you, but I checked the dimensions in another source with dimensioned drawings, and the numbers you found in the JBM list match the source I checked. I think part of the answer to your question is in the thickness of the copper jacket and also the size of the hollow portion inside the tip. If these were solid bullets with only one material of equal density throughout and no hollow tip area, you would be right that the lengths look incorrect; however, when you include the geometries of the lead cores, copper jackets, and hollow portions it’s more complicated.

 

[Bart B.]

It depends on how much airspace there is inside the bullet.

And the larger diameter bullet of the same weight has more cross section area and therefore more drag.

Try this to calculate BC based on bullet weight and shape;

https://en.m.wikipedia.org/wiki/Ballistic_coefficient

With two chronographs, try this one:

http://www.sellier-bellot.cz/en/product/rifle-ammunition/ballistic-coefficient-calculation/