Then you did one of two things:
1. You used a bullet that was not properly sized to your bore.
2. You pushed them too hard for the alloy.
On both occasions I shot lead, cleaning was a PITA.
The first was a couple of boxes of Factory LRN .38's out of my .357 ....
The second was 50 rounds or so of a buddy's 200gr SWC .... Oregon Trail cast bullets ..... my notes say they averaged 892 f/sec out of his gun, which does not sound "hot" for a 200gr .45ACP ..... they were sized .452, same as my plated bullets are......
In both cases, the bore was a PITA to clean.