This is what I found with the 'rifled' Foster slugs. About any lead alloy with lead a bore when contacting the bore walls, if there is no lubricant. Gas blowby is also a cause, as its melts lead of the sides of the projectiles, they blows that ahead of the bullet, which irons it onto the bore.

This hot gas blowby action usually causes the lead to adhere quite firmly to the leangth of th bore with hard bullets. With soft bullets that finally obturate to seal, it happens closer to the breech, right at the throat in most instances, before the bullet completely fills the bore to seal off the powder gasses.