So I dm for a group of friends and one of them are doing a one shot I introduced my bard character on last session and showed off heat metal spell , the guy running the one shot has told me he's worked his one shot to make heat metal useless just to mess with me by making stuff immune to magic, 

My question is if something is imune to magic or resistantant if I heat up there armour surely that would still do damage to them unless there resistant to fire damage cause I'm targeting the armour not the monster so the armours not magicaly imune to magic? Or is it a case of because the monsters imune or resistant to magic I can't target his armour?

Tried looking this up but no avail for clear answer 

If the one-shot involves lots of stuff that are immune to magic, create a character that doesn't use magic.

A one-shot isn't a type of scenario where you want to play a character that is seriously hampered by a basic rule of that one-shot.

The armour being worn by somebody immune to magic might make their carried/worn equipment also immune.

If the DM doesn't want your Heat Metal spell to work on his monsters unfortunately there's nothing you can do but choose a new spell or character.