TS, it isn't so much that you can't it's that there is a possibility of lead building up and increasing chamber pressures to a dangerous level. The poly rifling tends to lead much quicker than a straight bore. George
When you fire lead in a polygonal barrel the bullets want to skid over the lands and lock up on two flats at the time and I have had to melt many laps out with a torch because when they lock up it is impossible to drive them out with a hammer! Hard lead will help but better to use jacketed but under some circumstances jacketed will do to same. The longer the bullet the more likely it will lock up. I am speeking of rifles but am sure that pistol barrels will react the same way
