Belk, The bullet trap is a variation of the snail trap design. Mild steel because that is what I had. 45° back plate w/lengthwise sectioned pipe used to wrap the bullets around in the bottom so I don't have to use sand to trap the fragments like previous iterations. Sand is messy, dusty, & splashes everywhere!
Darkker, yeah, ¼" mild steel. Not planning to hit it w/true jacketed, nothing but low/mid speed speed lead. 22's & low speed .38 special WC's. & there are jacketed 17 & 22 rimfires, but WAY outta my league!
The question is because the fragments I saw were of the outside of the bullets, if you look at the close up of the single fragment showing the... can't be a cannalure, but looks like one, the core had stripped out of the outer shell, indicating to me the outside was much harder than the core. The inside of that shell shows the core had sheared/smeared as it proceeded forward thru the front of the bullet where it impacted the 45° from vertical back plate. I found no large pieces of the cores, it appeared the cores had hit, & gone forward to be fragmented (splashed?) into much smaller pieces. All the larger pieces were of the outer part of the bullets.
It makes absolutely NO sense to me that these lowly standard velocity bullets somehow have a much less ductile (tougher may be a better description) outer surface that would seperate on impact from a more ductile core! I don't get it. That is why I ask our experts. I have to ASSUME these are swaged from extruded lead alloy wire. Is there something in the alloy or the process that would cause the outer shell to become much tougher than the core? All my previous experience says the whole bullet typically "splashes" & fragments against the back plate. I know copper & zinc work harden, does lead (to such a degree it would be so obvious)? I don't think they were plated, but my first knee jerk reaction was "why do these simple bullets have an aluminum coating?" Not sure but I don't think you can basket plate aluminum. I don't have the tools to check density of such a small sample.
Maybe someone from an indoor range has seen this?