By inhaling a Metalun or its larger counterpart, Kirby becomes hard, rough, and impervious to nearly everything. The only downside is that Kirby moves very slowly and his flight is also greatly hindered. But just walking into enemies inflicts steady damage to them, without taking any himself. In terms of design and abilities, Metal is comparable to the Stone ability in Kirby 64: The Crystal Shards. However, in Kirby: Squeak Squad, Kirby is not invincible to bosses — they still hurt him.
Down B (Midair)
Kirby ground pounds. This is Metal Kirby's only means of damaging bosses before obtaining the Copy Scroll.
Requires Ability Scroll. Kirby transforms into a sphere. As a metal ball, Kirby rolls down hills and rams enemies & mid-bosses all the while.
Be careful. You can't get hurt, but don't get mad if you break stuff. It's not perfect, OK?
A Boost Orb called "Metal" appears in Kirby Battle Royale. When activated, it makes the player invincible, but heavier and with less mobility, alike to that of the Copy Ability. However, its effects are temporary, only lasting for 10 seconds.
Kirby of the Stars Copy Ability "Metal" Introduction Video