Custom VLD Bullet Accuracy?

[mmattockx]

Under the category of a little knowledge can be a dangerous thing, I have read Bryan Litz's book on ballistics and have been tinkering with designing new VLD bullets, based on his formula for calculating the i7 form factor. This is combined with reading the long running thread on custom turned solid bullets in the reloading forum.

I started with a .338 caliber bullet and have a shape that gives a G7 based form factor of 0.803 (G7 BC of 0.351 in 225gr. and 0.389 in 250gr.). It is not a crazy shape or anything and should be easily (if not cheaply) made on a decent CNC lathe.

My question is about accuracy. If I go out and have a batch of these made up, what are the odds that they will be decently accurate? Do bullet manufacturers spend endless months and years testing tiny variations to find the magical sweet spot, or is accuracy determined more by manufacturing processes in tight tolerances and bullet to bullet consistency than by small changes in the shape?

Thanks,
Mark

 

[rnbra-shooter]

Bigger bullets are more accurate than smaller bullets. So the fact that you're looking at a .338 bullet is better than if you wanted a .224 or .264.

The shallower the boattail angle, the more effect an imperfect crown has. With real match barrels this shouldn't be a factor, you ought to have a good crown.

The lower the density of the bullet material, the more problematic, because the bullet gets longer. I can't realistically see you using a material other than copper though (....?).

There is less science and testing that you might otherwise guess. A few different shapes might be tried, but you sure as heck don't get a 20th generation refinement.

If you're willing to describe the bullet I would be interested (but will understand if you choose not to).

But you asked, "what are the odds that they will be decently accurate?". I honestly don't know, though the more aggressive (drag-wise) a bullet is, the more worried I would be that it might be a very fussy, difficult-to-shoot bullet. You might want to ask Bryan for his SWAG on your odds.

 

[mmattockx]

Hey Daniel, thanks for the reply. Some more answers to your questions:

1) No material is chosen, but I can't imagine anything other than a copper alloy being a reasonable choice, as you say.

2) I don't have a length per weight, because the length doesn't figure into Bryan's equation for form factor. I will solid model the design to calculate length for 225gr. and 250gr. versions. Currently I have:

Calibre - .338 in
Ogive Length - 0.800 in
Ogive Radius = 4.00 in = 11.83 calibres
Rt/R = .556
Meplat diameter = 0.040 in
Boat tail length = 0.250 in
Boat tail angle = 7.5 degrees

This gives an i7 form factor = 0.803 based on Bryan's formula. The bullet will need some relief bands cut into the major diameter in order to function properly. On the Barnes TTSX bullets that Bryan has measured, these seem to add about 10% to the measured form factor compared to the calculated one. Adding 10% to my number gives me a form factor of 0.883. BC with this form factor is 0.318 for a 225gr. bullet and 0.354 for a 250gr. bullet. I forgot to add the 10% correction factor in the first post.

Mark

 

[mmattockx]

A screenshot of the solid model:

Length is 1.700" (same as the 300gr. SMK), weight as shown is 227gr. The relief bands are a WAG at 0.050" wide and 0.010" deep.

Mark