Polygonal Rifling is the only major development on the concept of rifling
ever since it was invented*. As we know, rifling is the machining of spiral grooves on a barrel
that will in turn impart a spin on the projectile
that moves along it. This spin
or twist rate
is measured in the distance a bullet would need to travel to complete one rotation on its axis, a twist rate of 1 in 16", for example, would mean that the bullet would rotate once for every 16 inches it travelled.
This spin that is imparted on the bullet creates gyroscopic forces which help to stabilize the bullet in flight and prevent it from tumbling.
The major improvement that polygonal rifling gives is a better gas seal and hence higher velocities while at the same time reducing bullet deformation and barrel wear. Some people will argue that this increased velocity comes at the price of reduced accuracy. This has yet to be proven. For what it is worth, an HK USP 45f placed on a ransom rest and shot at a target 25 meters away will consistently give 2.5" to 3" groups.
Ordinary cartridges can be used with the polygonally rifled barrels but note must be taken when shooting huge amounts of lead hardcast bullets. Please read shooting lead bullets in polygonally rifled barrels for details.
Determining if your barrel is polygonally rifled or not is easy. Field strip your firearm and look at the bore. If the profile of the rifling looks like it can be used as an allen head wrench it is polygonally rifled. If it has grooves but does not meet the aforementioned criteria it is a conventional land and groove rifled barrel. If there are no grooves then you probably are looking down a smoothbore shotgun barrel.
Popular pistols that have polygonally rifled barrels are the Glock and HK USP.
*If anybody has the date when rifling was invented please msg me. Also if anybody has a proven record of who invented polygonal rifling please also msg me about it. According to my research it first appeared in H&K's VP70
pistol which incidentally is also the first polymer pistol