If this "lead"came from a railroad, I would be afraid to use it for bullet casting for fear that it maybe Babbit bearing alloy.
Babbit is a bearing alloy that contains a lot of lead. It also contains a lot of tin and arsnic. If this is babbit and you have to use it for bullets do your smelting and casting outside because of the gray arsnic. It's very poisonous.
Babbit can have a hardness of up to 32 BHN. Hardcast bullets are usually no harder than 22-24 BHN. Pure solid copper bullets are also around 32 BHN so I wouldn't think the hardness of Babbit would be a factor in damaging a barrel.
If I had to use Babbit I would definately soften it with pure lead, but it would probably be my last choice for a lead source.