Barrel Twist Rate Chart, Rebuilt Around Bullet Length Instead of Bullet Weight

Don Miller's 2005 approximation replaced the older Greenhill rule, which was derived in the 1870s for lead cylinders and is genuinely unfit for modern boat-tail bullets. Miller wants six things, and only one of them is weight.

• ▸Bullet mass in grains.
• ▸Bullet diameter in inches.
• ▸Bullet length in inches — measured, not estimated from a catalogue.
• ▸Twist rate in inches per turn, converted internally to calibers per turn.
• ▸Muzzle velocity in fps, applied as a cube-root correction against a 2,800 fps datum.
• ▸Air density, entered as temperature in °F and station pressure in inHg.

The velocity term is weak — a cube root — so a 300 fps change moves Sg by only a few percent. The air-density term is not weak at all. Standard pressure at 6,000 ft is about 23.98 inHg against 29.92 at sea level, which multiplies Sg by 1.25. Cold does the reverse: at 10°F the correction factor falls to about 0.90 of the 59°F value.

That combination produces the single most useful rule on this page. A load that is stable in Denver in July can be marginal in Ohio in January. Spec twist for your coldest, densest expected day at your lowest elevation. A barrel is a permanent decision; the weather is not.

The reference atmosphere for every sea-level column here is 59°F and 29.92 inHg. Altitude columns hold temperature at 59°F and drop station pressure to 23.98 inHg, which is a realistic summer day at 6,000 ft rather than a standard-lapse-rate abstraction.

Bullet lengths in the header are typical measured values for common examples of each weight. Different makes vary by 0.02 to 0.05 in at the same weight, which is enough to move Sg by 5 percent — measure yours if the answer lands near a band edge.

Four zones, and the boundaries are not arbitrary — they come out of how much yaw a bullet can carry before it stops behaving.

• ▸Sg below 1.00 — unstable. The bullet tumbles. You will see keyholed, oval holes in paper, usually inside 100 yards.
• ▸Sg 1.00 to 1.40 — marginal. It will hold together on a warm day and start throwing fliers when the temperature drops or you move to a denser airmass. Groups open up rather than keyholing, which is why this failure mode gets blamed on the ammunition.
• ▸Sg 1.40 to 2.00 — the target band. Full ballistic coefficient, tight groups, enough margin for a cold morning.
• ▸Sg above 2.00 — over-stabilised. Costs very little in practice at rifle distances and buys nothing. It matters at extreme range, where a badly over-stabilised bullet stays nose-high through the descending leg and gives up a little BC.

The 1.40 floor deserves emphasis, because the older literature quotes 1.20 and plenty of people still do. Bryan Litz's work moved the practical floor up to 1.4 on the evidence that BC itself degrades as Sg falls below that, even while groups still look acceptable. You lose drop and drift performance before you lose accuracy. Spec to 1.4.

Same arithmetic, applied to the chamberings people actually spec barrels for. The cold column is 10°F at sea level, which is the condition that quietly breaks marginal combinations.

Seven inputs: twist rate, bullet diameter, measured bullet length, bullet weight, muzzle velocity, altitude and temperature. It returns Sg, the band it falls in, and the minimum twist that would put it at 1.40.

Six of those seven you already know. The seventh — measured bullet length — is the one people guess at, and it is the one the formula is most sensitive to. Sg varies roughly with the inverse cube of length at typical proportions. Get the length wrong by 5 percent and your answer is out by about 13 percent, which is enough to move a load from comfortable to marginal without you noticing.

Bullet manufacturers publish length inconsistently and sometimes not at all. Hornady and Berger are generally good about it. Several major makers publish only weight and BC, which is exactly the gap this page exists to close.

Solid copper bullets are the biggest offender. Copper is roughly 20 percent less dense than a lead core in a copper jacket, so a monolithic bullet at a given weight is meaningfully longer than the lead bullet it replaces. A 62 gr copper .224 runs around 1.010 in against 0.906 in for M855 at the identical weight. On a weight chart they are the same bullet. In the formula they are two bands apart — 1.36 versus 1.88 at 1:8.

Steel-cored military bullets bend the rule the other way and for the opposite reason: the penetrator adds length faster than it adds mass. M855 and M855A1 both want more twist than their weight suggests, which is the real reason 1:9 barrels acquired their reputation for being fussy about green tip in cold weather.

Polymer-tipped bullets add anywhere from 0.05 to 0.12 in of length that contributes essentially no mass. The tip is there for BC, and it is bought with stability margin. Anything described as a long-range or high-BC design in a given weight class will want faster twist than the plain version of the same weight.

Subsonic loads are the fourth case, and the awkward one. Velocity enters as a cube root, so dropping from 2,350 fps to 1,010 fps only costs about 24 percent of Sg — much less than people expect. But subsonic bullets are usually very long for their caliber, and length is the term that punishes you. A .300 BLK barrel is twisted 1:8 for the 220 gr subsonic, and the 110 gr supersonic then arrives over-stabilised at Sg 5.7. Nothing to be done about it, and nothing much lost either.

For handloaded ammunition this is trivial — measure a component bullet with calipers, tip to base, and you are done. Factory ammunition is harder, because pulling a round with an inertia hammer deforms soft tips and gives a slightly short reading.

Two workarounds. Pull with a collet puller rather than an inertia hammer, which grips the ogive and leaves the tip alone. Or measure a case-neck-to-ogive comparison against a known bullet of the same profile, which gets you within about 0.01 in and costs nothing but a round.

For polymer-tipped bullets, measure to the tip and not to the meplat. Miller's length term wants overall physical length. The tip is aerodynamically almost inert but it is still part of the length that has to be spun.

• ▸Measure three bullets from the same lot and take the mean — lot variation of 0.005 to 0.010 in is normal.
• ▸Check diameter too. Nominal .224 bullets run .2240 to .2245, and the cube term on diameter makes this worth a caliper reading.
• ▸Record the length alongside your load data. You will want it again in two years when you spec the next barrel.

Sg above 2.0 gets treated as a fault in forum arguments. It very rarely is. Faster twist than necessary costs a small amount of BC at long range because an over-stabilised bullet does not track the trajectory perfectly through the descending leg, and it slightly increases the odds of a thin-jacketed varmint bullet coming apart at high velocity. Neither effect shows up at 300 yards with a normal bullet.

What over-twisting does buy you is options. A 1:7 .224 barrel shoots 40 gr fine — Sg 4.5, no consequence — and it also shoots the 90 gr that a 1:9 barrel will never stabilise. If you are unsure which bullets you will end up running, twist faster than you need. The penalty for excess is negligible and the penalty for insufficient is a barrel you cannot use.

One exception worth naming: extremely light, thin-jacketed varmint bullets driven very fast from a very fast twist can and do disintegrate in flight. A 40 gr .224 at 3,900 fps out of a 1:7 sees roughly 400,000 rpm. That is the single combination where you should back off the twist rather than the velocity.