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Will M193 be accurate in a 1:7 or 1:9 twist barrel?

Introduction


It may be marginally less accurate due to the fast twist rate, particularly in 1:7 twist barrels. Unless you're trying to use these rounds for benchrest shooting, though, it shouldn't be enough to matter.

A bullet's flight is disrupted slightly as it leaves the barrel and after traveling some distance, will "settle down" into an even spiral, similar to a thrown football. The faster a bullet is spinning, the longer it takes to settle down. The most accurate twist rate for any length of bullet will be just a bit faster than what is required to stabilize it for its entire flight path (1.3 SG). But note that bullet quality plays a much bigger part in this equation. A uniform bullet will spin true; a non-uniform bullet will wobble and be inaccurate. As a general matter when shooting M193 or M855 (as opposed to match ammo) its better to err on the side of a faster twist rate. Regardless, both 1:9 and 1:7 twists seem to shoot M193 and M855 very well.

FACTS:

  • M855/SS-109 often has worse "wobble problems" because of the complex construction of the bullet. It's hard to seat the steel penetrator in the M855/SS-109 round exactly in center of the projectile. Most plants have good quality control for these rounds and spin them up in a balance test before sending them out the door but M193 and other simple cored rounds are usually more uniformly balanced.
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