The gunk. If it is lube, it is good gunk and ready to help the next shot. If there is no lead accumilating, don't clean it out unless you see accuracy start to degrade.
To get the best idea of how accurate the gun is. rest the barrel on about anything. Hold all the guns weight in your hands just as if you were shooting free hand, but let the barrel touch lightly enough so it stops all wiggle. This kind of hold lets the gun move exactly like it would when shooting free hand, but allows you to get a very precise aim. You'll find point of impact to be exactly the same as when free handing, which means you can use this hold on game in the field, resting on anything handy. Padded rock, branch, fencepost, even a tight fence wire fairly close to a post. Just don't let the side of the barrel touch anything.
Try it and your test groups will shrink and fliers dissapear. Then you'll soon notice that your lapped barrel is printing in the same place relitive to the sights, every time. Then you'll begin to notice what you did wrong when it doesn't, and correct it without any other teacher. Then you'll notice that your shooting is improving faster than at any time in your life!
If a pistol can't throw anything but buckshot groups no one can learn how to shoot it well, but if the gun is capable of putting all shots in one hole, it will teach you something about how to shoot! And there is 50 years of experiance in a nutshell!