FMJ yaw and upset performance

[JASmith]

...someone ... has expressed a view that there could be problems in designing a lead-free 6.5mm bullet of 120-125 grains with the required ballistic coefficient (.500+), since it would be very long, maybe creating stabilisation issues. It might therefore be necessary to drop the BC somewhat, in which case a larger calibre might better suited: it would keep the same overall length but with a lower LD ratio...

Tony,

Getting a bullet to tumble immediately on impacting a soft medium means that you will be working with one that is marginally stable in flight. I suspect you will have stabilisaton issues regardless of caliber.

To the best of my knowledge, the factors of concern for stability are the overall length, bullet weight (or density), velocity, and twist rate. Has anyone looked at the twist rate needed to stabilize a 123 gr solid copper bullet in 6.6mm? There's at least one on-line implementation of the Greenhill formula.

The other, perhaps more challenging, issue is the space taken up bu the long bullet. This favors the smaller bullet.

We can arbitrarily add length to account for voids, trace materials, etc. if it's deemed necessary for the yaw performance and compute the twist needed.

Barnes bullets offers a solid (no-lead) 110 gr bullet with an interesting ballistic coefficient:

.264" 110-gr Banded Spitzer BT SD=.225 BC=.452

They make no special twist rate call-out for this offering. One would think that this bullet is in the right ballpark.

Here's a couple of sources of very long solid bullets in 6.5mm. They might be able to give recommended twist rates based on actual tests:

http://www.barnesbullets.com/products/ri...ed-solids/
http://www.gsgroup.co.za/sp.html and,
http://www.thegunroomuk.com
http://www.barnesbullets.com/products/ri...sx-bullet/