Rammus W does not work against Heimer turrets
Rammus' W does not deal return damage to Heimer turrets, as well as Thornmail.
Rammus' W does not deal return damage to Heimer turrets, as well as Thornmail.
Heimer turrets deal magic damage.
Edit: That explain Thormail, idk about the Rammus W though.
Forgot to mention, the Thornmail bit would make sense pre-patch 5.16, but now that they've added extra scaling damage based on bonus armor, it could mean an exception to full magic damage autos.