Sorry, no, it's not "just a piece of metal." It's a deadly instrument that, if misused, can cause death or serious bodily injury. A car could also fall under that definition, but then, I don't lend my car, either.
Once, long ago, I let a "friend" ride my bike. He was gone a long time and finally returned. When I got on my bike to ride home, I immediately noticed the pedal had been bent, likely due to a crash my "friend" had. I mentioned it right away. Oddly, he didn't know what I was talking about. I suspect he tried some jumps/stunts that he didn't want to risk damagingHIS bike attempting.
I don't lend guns to friends. Not even my $20 Jennings.
I offer a time to go shooting together, so we can both have fun. If he can't go when I can to shoot MY gun, then that's too bad for him.
Don't feel bad or guilty about not letting your overly presumptuous friend borrow your gun.