When I started reloading I had problems with turning a rifled pistol barrel into a smoothbore within twenty rounds. And this was after I thought I'd done everything right. With the help of the gentlemen at www.castboolits.gunloads.com
and here I figured out the problem.
First make sure you are measuring your barrel slug with a micrometer and not a caliper. That small bit of inaccuracy in the caliper's measurement can make a big difference.
Second, measure the bullet diameter before loading it (as you have done) and then pull the bullet after reloading it into a cartridge. Often there is enough swaging of the bullet when being pushed into the brass and then crimped that the bullet ends up undersized. My bullets would start out at 1-2 thousandths oversize before loading and end up 1 thousandth of an inch undersize after being pulled, which means I had lots of hot gas spraying past the undersized bullet and causing leading.
The primary problem with leading is usually bullet fit/size. Other factors like lube type, lube groove type (ie, tumble lube vs. conventional grooves), bullet speed and hardness are usually addressed once bullet-barrel fit is optimal. To overcome my bullet swaging problem I just ordered an Accurate Mold Company mold (both .45 and 9mm and 44 special) at two thousandths over barrel diameter and they drop into my brass without sizing (usually at requested size to one-half a thousandths oversize) and leave no leading whatsoever in the barrel. So for my 9mm I use bullets at .357-.358, .45 at .453 and 44 special at .432-423". Good luck and check out the castboolits.gunloads.com site- it's a great site for reloaders.
best wishes- oldandslow