List of best twist rates for different bullet weights per caliber etc?

[pgg]

Anyone have a list like this showing best twist rates or range for different bullet weights per caliber? Thanks PG

 

[rubbershotgun]

like this one?

http://www.riflebarrels.com/faq_lilja_rifle_barrels.htm#twist%20rates

 

[SS427]

Berger lists minimum twist rates for given bullet weights. BIB bullets recommend ideal twist rates for his bullets. BIB doesn't offer nearly as many calibers and bullet weights though.

 

[artbosco]

Anyone have a list like this showing best twist rates or range for different bullet weights per caliber?

SHILEN's got this chart: http://www.shilen.com/calibersAndTwists.html

 

[Laurie]

PG,

remember bullet weights are not the key criterion, rather it is bullet length in terms of calibres - the longer a bullet is in relation to its diameter, the faster it needs to rotate to be stabilised adequately. Weight can be used as a guide as the heavier a bullet, generally the longer it is. However, we live in a world where a 180gn 308 for instance may be purchased as a short, blunt flat-base round-nose design, or as a VLD that is half as long again and needs a much faster twist rate to be stable. We also have an increasing number of no-lead bullets and since copper is lighter than lead, a 150gn Barnes TSX is substantially longer than a 150gn conventional lead-core job, so needs a faster rifling twist.

MV has an effect, but a relatively small one. Atmospheric conditions affect stability too, especially when the bullet is only marginally stabilised by the rifling twist being used - OK at Raton 6,500 ft ASL in dry warm summer air; no go at below freezing temperatures mid winter in dense damp air on the Atlantic seaboard.

The easiest way of finding the Sg (coefficient of stability) is Don Miller's Twist Rule downloadable as a little ExCel program from here, elsewhere on the AccurateShooter website:

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/


but .... you do need to know the bullet length. The best source for most match bullets if you don't have one on hand to measure is Bryan Litz's book Applied Ballistics for Long Range Shooting advertised on this site, or ask other forum members if they can measure one and let you know.

A more advanced programn is availbale on the UK barrelmaker Border Barrels website, but you need still more information re nose and boattail lengths (also available for most match designs in Bryan L's book):

http://www.border-barrels.com/external_ballistics.htm

Any really serious long-range target shooter should read Bryan's book anyway incidentally, also anybody else who wants a layman's guide (no algebra!) to bullet design, characteristics and performance, external ballistics and more.

In the absence of detailed bullet information, you have to use a weight / calibre / twist-rate guide like that on the Lilja site, but it cannot be treated as definitive. For instance some 0.224" 75gn match bullets will stabilise over most distances at .223 Rem velocities from a 1-9" twist barrel, but others (VLD types) likely won't, and even if they do OK in 100yd testing may perform poorly at long ranges with poor elevation consistency on the target.

 

[RonAKA]

I find the easiest is to just check the Berger Data Sheet. Link below. There are enough dimensions to calculate stability in a slower than recommended twist if you go to the Border Barrel site that Laurie posted a link to. I use 9 degrees for a boat tail. At the Border site be sure to input your actual twist if you want to see the stability vs muzzle velocity chart. Note that this is only muzzle velocity not down range velocity. Unless you go subsonic, stability increases as the bullet goes down range because forward velocity decreases faster than spin rpm.

As for ideal twist the minimum that stabilizes the bullet is the ideal. Faster just gives it unnecessary wobble unless the bullet is perfectly balanced. Ideals may be slower than the minimum recommended by Berger, but the margin of safety for a temperature drop or atmospheric pressure increase will be reduced.

Berger Data Sheet

 

[pgg]

Thanks guys. PG

 

[Laurie]

Ron,

that's a good data-sheet that I'd failed to notice. Berger has all sorts of stuff that doesn't always show up - I reckon their site could be better organised Or ..... maybe, it's just me!

 

[SS427]

I agree Laurie. I've been all over the Berger website and never discovered that data sheet. Thanks for posting it Ron.