What is bullet slump?
When the bullet starts down the barrel, it accelerates to roughly 65-85% of its full velocity in the first few inches of the barrel and depending on the burn rate of the powder, so the base of the bullet with the gas column behind it will compress until it has nowhere to go but down the barrel. The nose section will resist the initial acceleration and the bullet will distort, shorten, or slump. With pure lead and softer alloys, this is a problem and can be a cause of instability. Slump can be controlled by designing the bullet with as much support as possible, and/or a harder alloy. Generally speaking, slump is not a problem with alloys of 1-20 lead/tin and harder.
