Polygonal Rifling
A rifling profile that uses rounded hills and valleys instead of sharp lands and grooves, giving a tighter gas seal and less fouling.
Polygonal rifling replaces the squared edges of conventional lands and grooves with gentle, rounded curves that form a many-sided polygon when the bore is viewed in cross section. The bullet still gets engraved and spun, but the smooth contour grips it without the sharp corners that scrape and shave jacket material. That softer engagement tends to seal propellant gas a little better behind the bullet.
The rounded profile leaves fewer sharp pockets for residue to collect, so shooters often report reduced fouling and easier cleaning. The approach is most familiar on certain pistols, where some makers built their reputation on it, and it appears on select rifle barrels as well. The trade-off is that severely worn polygonal bores can be harder to measure for groove diameter than conventional ones.