AR-500 is going to be your best bet. It is WAY harder than mild steel, which will make is much easier to transport (cause it doesn't have to be as thick) & it takes a LOT more abuse. AR-500 will cause bullets to splash rather than indent (most of the time). Mild steel is usually cheaper but requires much thicker material to withstand impacts. IIRC a member here sells AR-500 targets, I still use the ol' standby run 'o' the mill oilfield blind flanges. They can be had pretty cheap so they are somewhat disposable + I have access to a couple hundred thousand dollars worth of welding eqipment right outside my office

so repair is pretty easy! lol On the negative side, these pigs are VERY HEAVY (for a target), got to be getting up there close to 100lbs. Here is a pic of one of my targets @ 550yds, getting worked over by my 6-284: