[MicroBalrog]

I really don't know *** this is:

What's a "polygonal barrel" (relates to shapes of grooves in it AFAIK) and what is it good for?

[Destructo6]

Instead of the lands and gooves found in typical barrels, think of a polygon, a hexagon or octagon, twisted 360 degrees every 10 inches or so.

It's suppossed to offer similar accuracy, with less fouling and perhaps a better gas seal. That's my understanding, anyway.

IIRC, the Uzi went to a polygonal bore for a secret service contract, which they didn't get.

[SDC]

Also seen in Glock and H&K firearms; here's a page with pictures that might be able to give you a better idea of what's meant http://www.firearmsid.com/A_bulletIDrifling.htm
HTH.