Yep, polygonal rifling. It has multi sided (hence polygonal) lands that allow them to be much shorter than traditional lands. This means the groves are shallower than normal. The benefit is a tighter seal around the bullet as it is pushed out the barrel which means more velocity.
The down side is lead builds up and fills the groves quickly and makes it too tight for the bullet. Once it is too full of lead, the pressure has to go somewhere else so the chamber may crack. Traditional grooves are deeper and lead build up takes a lot longer before there are pressure issues.