Viper, unless the M-15 is like new has box and papers $500.00 is a LITTLE to high in today's dollars, but in 6 -18 months who knows.
Now the real issue, it sounds as if your friend, sic, is unable or unwilling to repay the loan. So take the revolver and drop the friend, if he was ever one. Ya don't need folks like that in your life.
Good luck whatever you chose to do.