Tim Sundles of Buffalo Bore fame has had this on his website for quite a while now . . .
HARD CAST BULLETS IN POLYGONAL BARRELS
THE SOURCE OF THE MYTH
True hard cast bullets that are properly lubed (as opposed to lead swaged bullets)
will not lead foul polygonal barrels any more than any other type of rifled barrel. Ever since Glock Corporation warned to not use lead bullets in their pistols with polygonal barrels, a myth that hard cast bullets will lead foul polygonal barrels has become widespread in some parts of the firearms world. However, the myth is untrue. Hard cast bullets are not "lead" bullets. (See my article on "Dangerous Pure Lead Cowboy Bullets")
Folks (especially folks involved in the shooting industry)
have a strong tendency to generalize most everything……if you do this, you’ll end up with generalized results as opposed to exacting results. Because hard cast bullets are grey in color, does NOT make them “lead” bullets.
Pure lead or nearly pure lead bullets have a tendency to foul any barrel, not just polygonal barrels. Years ago, when several Glock pistols experienced cracked barrels because of lead fouling build-up from shooting pure lead bullets, Glock issued a warning not to shoot lead bullets in their polygonal barrels. From that warning, the myth that you should not shoot hard cast bullets in polygonal barrels was born. This myth was born because folks see that grey colored hard cast bullet and ASSUME it has anything to do with pure “lead”, which it does not, at least when it comes to characteristics and dynamics that make bullets perform!
HARD CAST BULLETS DO NOT FOUL
Provided you use real hard cast bullets, that are properly sized and utilize good lube, you can shoot them all you like in polygonal barrels without causing lead fouling deposits at the front of your chamber or anywhere else in the barrel. I have fired literally thousands of properly alloyed, lubed and sized hard cast bullets from my various Glock pistols and have never experienced any metallic fouling build-up of any consequence. If I did that same amount of shooting with cheap pure lead swaged bullets of unknown origin, I could have severely lead fouled my barrels, to the point of creating a bore obstruction and possible cracking or bursting a barrel. However, in my experience, quality hard cast bullets won't foul a Glock polygonal barrel or any other type of barrel but lead bullets normally will.