That looks like powder blow-by that the bullet has compressed into the rifling. Check diameter of bullet and ACTUAL measured groove diameter. For FMJ, they should be the same or slightly larger. Next, shoot about 500 and I'll bet the condition stays the same and does not get worse. For "soft" lead bullets (8-13 BHN) you should shoot bullets that are at least 0.001" larger than actual groove diameter. With HARD lead bullets (18-22 BHN), i find at lest 0.002" larger than groove diameter works best for accuracy.
You should seat a few bullets, crimp, and then pull them and see if there are crimp rings or a change in bullet diameter. The taper crimp does NOT mean the case mouth needs to contact the bullet, it just removes case mouth flare.
Exposed lead at the bullet base CAN NOT be a source of leading. Lead has a high heat content and will not melt from the flame front in the microseconds it is exposed.
Old wive's tales die hard, particularly on the internet.
Decades ago, one of the writers went and put Templ-marker or some such temperature indicating crayon, on the bullet base, loaded and fired the bullets, recovered the bullets and the wax didn't melt and there was no temperature change indicated.