I shot by pistol last night and remembered to take a few pictures before I cleaned it. I think it should help visualize what a constriction does to a lead boolit. Not the best pictures, but should be good enough.
Notice there is no lead around the forcing cone and into the bore for a little way. That is where the constriction is. Then, once the boolit clears it, there is a slight moment where there is enough space for the gases to get past the gas check and cut lead off the boolit. Then, when the boolit makes it a little further down the bore it seals once again and there is no more gas cutting.
The picture taken from the muzzle end doesn't show the lead near the forcing cone as well, but it should give you a general idea of how slick the rest of the barrel is. I did not measure just how far the lead cutting happened(maybe I'll remember to do that next time) but if I was to guess maybe between 1"-2".
I think, but will say its just my thought, that if I had a softer boolit then I might not get that leading as bad as I am. That's my theory anyway. The gas check should be doing its job to stop gasses from cutting by, but maybe because the boolit is so hard it does not "slip" past the constriction, but rather "pops" past it. If that makes sense. Does in my mind.