Choosing a twist rate for a custom barrel

Written by a former custom barrel maker

Twist rate is the distance the rifling takes to make one full turn, written as 1:8 or "8 twist" (one turn in 8 inches). Faster twist (smaller number) spins the bullet faster. The point of picking a twist is simple: spin the longest, heaviest bullet you plan to shoot fast enough to fly stable.

It's about bullet length more than weight

Stability depends mainly on how long the bullet is. Pac-Nor's twist guidance puts it plainly: choose a twist "sufficient to stabilize the longest and heaviest bullets you plan to use." Lilja's FAQ notes that solid copper and other monolithic bullets are longer than lead-core bullets of the same weight, so twist charts built on lead-core bullets can be too slow for them.

Run the numbers before you order

Use a stability calculator with your actual bullet's length. Berger's twist-rate stability calculator reports a gyroscopic stability factor (SG), and Berger's guidance is to aim for an SG of 1.5 or higher for full performance. Altitude and temperature change the result, so run it for the conditions you shoot in.

Shop-floor rules of thumb

Pick your bullet first, then the twist. Not the other way around.
If you're between two twists, the faster one usually covers more bullets. But don't go faster than you need with thin-jacketed varmint bullets without checking with the bullet maker.
Check which twists the maker actually stocks tooling for in your caliber. A twist they don't run often can add to the wait.

For specific numbers, start with the twist charts barrel makers publish for each caliber (for example Pac-Nor's caliber and twist chart and Lilja's twist table), then check your exact bullet in a stability calculator and with the bullet maker's own recommendation.

Sources

This is general information, not gunsmithing advice. Have a qualified gunsmith fit and headspace any barrel.