I think that we should remember that even in the real world, there are people that are able to pull trucks with ropes without much formal training. They are not by any means classed as superhuman, but merely as strong. That's where I believe that the likes of Mr. Satan would go.
Goku and Kuririn would in the beginning of Dragonball go as at borderline superhuman. A huge boulder such as the one they pushed could as well be as pulling a truck without wheels (why I class them as stronger than the strongest of this world's humans). But IMO at the 22nd Tenkaichi Budokai, that's when we can class them as superhuman.
Mr. Satan, albeit in filler only, does perform such acts as ripping a phone book in half, or pulling a truck, further emphasizing about the extent of his power; still in the realms of this world.
His punch through the wall of the truck? Meh, lets say it was a thin wall.
And hitting through bricks, we have humans
who can perform similar acts already.
So it makes sense that Mr. Satan wouldn't be able to survive a gunshot.
About Kuririn, I don't believe that he was much weaker than the older students of the Temple. I believe that he started out weak, but became stronger with time. But as the bullying started early, and the other students kept improving, his self esteem stayed low even though he became stronger. So they could bully him without any resistance, really. It was a self esteem issue IMO.
