Leading:
You hit the loud button, BANG, the bullets of to the races. The 1st thing the bullet has to do as it moves forward is seal the leade of the throats in the cylinders. Same bullets with different seating length/oals. The top bullet has a small gap around the nose, the bottom bullet is a tight fit in the leade.
If the leade/throats of the cylinders are smaller then the bore diameter, you'll get leading.
As the nose of the bullet enters the leade, the back/base/bottom drive band of the bullet has pressure exerted against it. That pressure is in the forum of hot gasses. That pressure not only pushes the bullet forward, it expands the bullets base (obturation) to seal the same leade/throat of the cylinders. The bullet enters the forcing cone & then the bbl. The bullets base of the bullet has to now seal the bbl. As the hot gasses put pressure on the base of the bullet it is also compressing the grease groove of the bullet. This compression of the grease groove forces the bullet lube outward & forward. Years ago on another website I was talking with a member about the differences between square lube grooves Vs round lube grooves. He sent me this picture, if you look closely you can see that the lube grooves of the recovered bullets are shorter/compressed.
Another example of the bullet's lube groove compression. This is a hollow based hollow point cast lead swc.
As the hot gasses expand the bullets base & compress the lube groove, some of the hot gasses get by the bullets base and into the lube groove. This pressurizes the lube groove and aids in pushing the lube outward along with forward lubing/sealing the middle/front drive bands. The lube as it is pushed forward will also seal any micro imperfections in the bbl.
Man/moment/machine time:
Have the right load (pressure) for the alloy of the bullet so that pressure can expand/seal the bullet's base, compress the lube groove and pressurize the lube groove. Get all these things correct and you will not have any leading.
Most leading occurs from using too hard of an alloy & flame cutting occurs to the bottom drive band and causes leading. Most commercial casters use to hard of a alloy/too hard of a lube. +/- 15bhn bullets doing 260fps+ in a 308w.
What the bore of that 308w looked like after 30 shots with that 2600fps+ load/15bhn lead bullets. A lot of people mistake antimonal wash for leading. The grey you see in the bbl easily comes out with a single pass of the brush.
Same bullet cast the same day as the lead bullets used on the target above. This time they were PC'd (coated) & shot from the same firearm/same load/same day/same everything. Note the same accuracy and yet there's 50fps when using the coated bullets.
This is why coated bullets are superior to their traditionally sized/lubed counterparts. You have to get the alloy perfect to the bullet design for the pressure of the load to be able to seal the bbl. The coated bullets already have the lube everywhere it needs to be and can do so with a lot wider range of alloys Excellent fit/excellent seal/excellent obturation leads to higher velocities and makes it easier to find accurate loads.
If you are not sure about your bullet/bullet design/alloy/lube, simple take Johnsen's paste wax and coat the heck out of the bullet's body. This will take the pressure of the load/correct alloy for the pressure/correct lube for the pressure out of play. Hence, seal the bbl just like coated bullets to stop the flame cutting that causes leading.