Bullet Stability

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pendennis

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My previous experience in long-range shooting (300 yards plus), except for service rifle competition, has been limited to muzzleloading and blackpowder cartridge. These were all .45 caliber rifles. I used a Tryon Creedmore, an Enfield Volunteer, and a Sharps 1874 (chambered in .45-120).

A lot of concern, especially with heavier bullets (450gr, and heavier), centered around stabilizing the bullet. Most of the time these heavy bullets would still be yawing and pitching at the 200-yard mark. They would begin to perform well past 200 yards.

This brings me to the question of bullets for my .308 rifle. Our range is limited to 200 yards, so practicing for 300 yards and further is mostly limited to elevation.

Do the heavier bullets for .308, say 175 grains and heavier, have stability "issues" at nearer ranges (200 yards, or less)? Do they gain stability as they head down range, or are they as stable as they will get as they exit the muzzle?
 
With tangent ogive bullets with fairly short (around 7-calibre) radius ogive-tip sections, definitely no. So, many popular and proven shorter-range performers in the .308W such as the 168gn, 175gn, and 180gn Sierra MatchKings will usually shoot very well at 100 and 200yd, all other things being equal. They are short on BC compared to many competitors and the 168 plus 180gn models become unstable somewhere around the 800-900yd mark at 308 MVs, but neither of these factors matter at 200yd unless your range is situated on a seaside cliff-top that gets gusting Force 10 gales! Contrary to common belief, they do not need very fast rifling twists for this type of shooting, the commonly found 1-13" in Palma, Fullbore and Target Rifle barrels being fine even for the 180 at shorter ranges in all but the densest and coldest air.

If the barrel likes them, 155s often do equally well or even better at these ranges in grouping terms, so don't discount them either.

VLD secant ogive type bullets may be a different matter. I say 'may' as there is a lack of concensus here. Some people are convinced they need 200, or even 300yd to 'settle down' in flight and tell you that 100yd group-testing of load combinations is a waste of time. Other people find they shoot very small groups indeed at 100yd, and I've got to add my smallest ever 100yd 6mm BR group came from a long Berger 105gn VLD out of a 1-8" twist barrel. My .223 Rem F/TR rifle's favourite load shoots 0.3-0.4" groups all day at 100 with 90gn Berger VLDs out of a 1-7" twist barrel. So, there's no black or white situation here.

If limited to 200yd, I'd never choose a VLD bullet anyway, so my advice is try a mix of 155s, plus the 168 and 175gn SMKs and just use whichever gives the smallest groups on a consistent basis.
 
You're lumping a lot of things into one question. Just because a bullet has pitch and yaw doesn't mean it isn't stable. Moreover, there is both primary and secondary pitch and yaw. Increasing the rate of twist means, as a general rule, and at the muzzle, that one goes up, and the other down. Most bullets will damp primary pitch & yaw by 200 yards.

As far as bullet "weight" goes (length is the most significant factor), you need a Gyroscopic Stability of about 1.4. If your .308 has a 10-twist barrel, you're good to go with just about any jacketed bullet. If it is a 13-twist or less, you should probably limit yourself to bullets about as long as the SMK 190s.

There are a number of good books on the subject, but the math does get rough as the explanations get detailed.

You might enjoy

http://riflemansjournal.blogspot.com/p/articles-index.html

Good luck to you
 
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