FULLBORE Bullet Analysis

B

BryanLitz

Industry Expert
Rating - 100%
1   0   0
Location
Cedar Springs, MI
I've posted a new article to my website titled: 2009 FULLBORE Bullet Update

This article provides analysis of 8 different 30 cal 155 grain bullets that are commonly (and uncommonly) used in Fullbore competition.
The article starts by providing the tested BC's for each bullet, then goes on to analyze the effect of the different BC's on wind deflection and score for 1000 yard (US) prone and F-class targets.

Analysis is provided on MV and performance from service rifles, which will also be relevant to those who use shorter barrel bolt rifles. Stability requirements for this current crop of FULLBORE bullets is also discussed.

There is no mention of the bullet's terminal performance. The article addresses the external ballistics specific to Fullbore competition.

Enjoy,
-Bryan

PS, Please let me know (bryan.litz@appliedballisticsLLC.com) if you notice anything 'fishy' about the .pdf. Sometimes the font goes haywire, don't know if it's a browser problem, embedded fonts, etc. Thanks.
 
Great read thanks for sharing.

A quick question on your twist rating in the article does the hollow cavity in the nose of the bullets affect the twist rate that can stabilize bullets. It would seem that from your books pic pg 150 the Cg and Cp (were all internet experts now LOL) would be closer together on the Lapua and Berger 155.5 because of the large hollow cavity that have been built into the bullets to reduce drag?

The reason for the question is that the folks in the UK are running 13.5 14 twists and are having no difficulties stabilizing the 155s

Caveat I do understand that 1.4 Sg is for all conditions (read season), but being a fair weather target shooter my F/TR rig rarely sees temps below 40oF

Many thanks
Trevor
 
Trevor,
Thanks for your comments.
The Miller stability prediction formula is semi-empirical, meaning it's based on experimental results, assuming a bullet of 'nominal' construction. A larger than average nose cavity changes the balance, and makes the bullet a little different than average.
The CP will be unaffected, because it's determined by the exterior shape of the bullet only.
The CG will be shifted aft compared to a bullet with a greater serving of core.
The result is greater separation between CG and CP, meaning that the formula would tend to over predict the actual stability of the bullet (the real bullet would have less stability than predicted because it has more distance between the CP and CG than the formula assumes).

However, on the other hand...

Another property of the bullet that's related to it's stability is the ratio of axial and transverse moments of inertia (not covered in the book :) ). A lower ratio makes for an easier to stabilize bullet. A bullet with a short core will result in a lower ratio than a bullet with a long core, thus improving stability beyond what the formula would predict.

In the end, the larger than average void in the nose has two offsetting effects related to stability. Which is more dominant? It's hard to tell, but I would bet their close to equal. That's part of the beauty of Don's formula, the outliers tend to have offsetting effects which result in low net error for a wide range of bullets.

You may get away with a 1:13.5" or 1:14" twist for the longer bullets, but according to the stability formula, you're reeeeealy pushing it! (especially with the Scenar) I'm much more comfortable with a 1:13".

Take care,
-Bryan
 
Bryan,

Do you have any thoughts on the effects/benefits of using "tight" bores with these sorts of bullets? I have tended to consider this preference to be somewhat anachronistic but it is still widely used.
 
O,
Some have speculated that the shorter bearing surfaces like we see on some of the new 155's (like the Berger Fullbore bullet and the new Sierra) are better in a tight bore. Don't know of anything definitive. Those short bearing surface bullets seem to shoot just as well out of standard bore/groove dimensions and even rough factory barrels.
-Bryan
 
You must log in or register to reply here.
Back
Top Bottom