The AR15.Com Ammo Oracle

History and Basic Design of .223 and 5.56 Ammunition.

Performance of .223 and 5.56 Ammunition.

Terminal Performance of .223 and 5.56 Ammunition.

.223 and 5.56 Ammunition Testing

Selection of .223 and 5.56 Ammunition.

Ammunition recommendations from the authors of the AR15.com Ammo-Oracle.

Purchase and Storage of .223 and 5.56 Ammunition.

Legal questions.

Miscellaneous .223, 5.56 and Other Ammunition Questions.

Ammo Oracle

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

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.

Fact: 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.