A 7.62x39 HP(copper-plated steel over lead) leaves a surprising large. jagged hole in sheet metal. I don't know if you consider 2000 fps "high velocity" or not, but it's certainly an ubiquitous round.
Maybe, but that's muzzle velocity. With a ballistic coeficient of roughly .260 a 7.62/39 round will be travelling about 17-1800 fps and have about 800 ft/lb energy left at a hundred yards (pretty long shooting against a moving target). That's approaching what I would consider low velocity for a rifle round. Still, at 800 ft/lb energy, I would expect to see an exit hole in the light weight sheet metal and the plastic interiors used in automobiles today. Sheet metal usually doesn't cause expansion in a HP (and most of the combloc HP stuff almost never expands anyway unless you manage to get your hands on a true hunting round). I suppose it's possible that the shooter might be firing from a greater distance, but in that case, it would either require a quite skillful shooter (70 mph moving targets from a distance aren't all that easy) or he's missing a lot and just getting lucky someitmes. If he's firing a lot of rounds and just hitting occasionally, I would think that someone would notice the gunfire (of course I'm unfamiliar with the local landscaping, but I'm assuming some degree of development or population within several hundred yards of the highway). I'm still thinking a 9mm handgun round (from either a handgun or carbine) or a smaller caliber low powered rifle round (.22 mag possibly if close, but definitely not a .22 LR., maybe a .30 carbine class of round). Of course, speculation from a single picture of a single bullet hole is not exactly worth a whole lot of anything other than interesting conversation. Certainly nothing to even think about drawing a credible conclusion from.
I like an SKS for rough handling in crud and mud.They make real good kids guns,too.They don't kick much but still throw enough lead for a deer if the range isn't too big a factor because of the low velocity.
A 30-30 Winchester is right about the same thing as a 7.62x39 for power,to my way of thinking but would be a flat or round nosed bullet.A silvertip cartridge or similar could cause what we see in the pictures.
Also,if the sighting of juveniles accounts for anything,a 30-30 Winchester used to be most kids second rifle after a .22 and there are a lot of them around.
I didn't word that very well at all but you get the general idea.Maybe I can square my thoughts out when I'm awake(or trash them altogether).:)