So, I just added the contact damage immunity. I noticed I was immune to contact damage in the forge, but ran out of enemies to test it on. So, in bullet hell, I finally confirmed it was this item. And yes, I tested the gun knight armor and gauntlets first, as well as the grappling hook.
My Gear: Pilot -
Guns: Rogue Special, staff of firepower, mutation, winchester, corsair, grenade launcher, colt 1851, fossilized gun, frost giant, flare gun, smiley's revolver, mega douser, the judge, abyssal tentacle, klobbe
Actives: grappling hook, napalm
Passives: disarming personality, hidden compartment, ring of chest friendship, blast helm, amulet of the pit lord, junk x2, gun soul, heart of ice, heavy boots, master round II, master round III, gun knight gauntlets, gun kinght armor, charming rounds, gungeon blueprint, hip holster, hungry bullets.
I'd like some independent confirmation, because when this test was finally successful, it was my last heart, it triggered gun soul, and the rat took the helmet. So, unless I see the helmet on another run soon...