there is a 20c. main battle rifle (g3) that used a polygonal rifling system. and a well-known austrian plastic pistol manufacturer still uses it. some famous german machine guns ( ww2 vintage mg42) used a polygonal rifling system. see this linky
This Linky
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Originally Posted by this linky
A number of advantages are claimed by the supporters of polygonal rifling. These include:
Not compromising the barrel's thickness in the area of each groove as with traditional rifling.
Providing a better gas seal around the projectile as polygonal bores tend to have a slightly smaller bore area, which translates into more efficient use of the combustion gases trapped behind the bullet, slightly greater (consistency in) muzzle velocities and slightly increased accuracy.
Less bullet deformation, resulting in reduced drag on the bullet when traveling through the barrel which helps to increase muzzle velocity.
Reduced buildup of copper or lead within the barrel which results in easier maintenance characteristics.
Prolonged barrel life.
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whitworth projectiles were also used in hex rifled cannons in the civil war by the confederates. 70 pounder:
12 pounder whitworth as used by the CSA
note that it was a breech loader.
projectiles:
