Slingshots are very dangerous when the elastic breaks and smacks back into your hand. That is why I went to guns.
Pits occur a lot like cavities in teeth. A small surface defect starts, material below the surface can be lost. Never a straight line like a drilled hole. Sometimes you get a cone shape defect which is larger than the pinpoint on the surface sometimes it is wider at the top than at the bottom. With the pinpoint at the top defect the surface gets very weak and then fails to expose a larger than expected pit. I have seen pits that were smaller at the deepest part and some which were larger. Rust will follow the pathway of least resistance so in internal defect or difference in metal will change the shape of the defect. I have seen a set of twist barrels with a defect that was between two layers and was at an angle to the bore. Pits can be weird like that.
So given that you screw will give you more information use it as another tool to evaluate the pit. Just take it with a grain of salt. Metal defects like pits are more about location than depth sometimes. A ported barrel, which is just a pit that was through and through is safe at 28" but not one I want to risk closer to my hand or face.
