Rook's (Ben 10) Proto Tool can turn into a blaster, power sword, quantum staff, bow, grappler, shield, fishing pole, and numerous other equipment.
King K. Rool's Blunderbuss (Donkey Kong Country 2: Diddy's Kong Quest/Smash Bros) is a regular blunderbuss that also...
Samus Aran's (Metroid/Super Smash Bros.) Paralyzer is a stun pistol that can convert into a plasma whip.
Labrys (Persona 4 Arena) wields a massive battle-axe...
...which can also become a jet-pack.
Ruby Rose (RWBY) wields Crescent Rose, which can become a scythe...
Whisper the Wolf (IDW's Sonic the Hedgehog), wielding a Variable Wispon
, allowing it to switch functions and use a variety of different Wisp Color Powers
Saki Amamiya (Sin and Punishment) wields the Dolphin Gun, a gun/beam-sword hybrid which he uses to slash and shoot with.
The Doctor (Doctor Who) has the Sonic screwdriver which can pick locks, detonate objects, melt ice, shut down electronics, and can be used as a weapon.
The Tool Gun (GMOD) has a wide variety of functions including changing the color of objects and summoning props.
Cull Obsidian (Marvel Cinematic Universe) wielded the Chain Hammer which could turn into a Hammer, a Pickaxe, a Shield, a Scythe, a Claw, a Vice, and a Gauntlet.
The Shiny Rod (Little Witch Academia) can turn into a Bow, a Ballista, a Ax, a Grappling Hook, a Sprinkler, and a Broom.
Daredevil (Marvel) wields his twin billy clubs which can turn into a nunchunk, staff, and cable with grappling hook.
Erza Bridger (Star Wars) uses a lightsaber that...
...can also become a blaster.
The Yuuzhan Vong (Star Wars) wield Amphistaffs which can spit poison/acid but can also be used as a whip, sword, staff, and spear.
Omni-tools (Mass Effect) can generate kinetic barriers, freeze enemies, fire electric blasts, become 3-D printers, and more.
Aang (Avatar: The Last Airbender) has a staff...
...which can also become a glider.
Sirica (Kirby: Right Back at Ya! ) wields a sword-like weapon that can turn into...
The Lawgiver (Judge Dredd) uses explosive, full metal jacket, incendiary, and armor-piercing rounds.
Jean-Baptiste Emmanuel Zorg (The Fifth Element) wielding the ZF-1, a gun that can turn into an Arrow Launcher, Rocket Launcher, Net Launcher, Freeze Ray, and Flamethrower.
The Widow's Kiss (Overwatch), a sniper rifle that doubles as a machine gun.
The Pyro’s (Team Fortress 2) Flamethrower can be used to set enemies on fire but...
...it also doubles as a reflector against non-bullet projectiles.
Donatello's (Nickelodeon's Teenage Mutant Ninja Turtles 2012) skills with his bō staff translate to his proficiency with the naginata. His staff possesses a retractable blade that is mostly used in cases when he is fighting robots.
Jayce (League of Legends) uses the Mercury Hammer which can become a Mercury Cannon for long range fighting.
The Lancer (Gears of War) can be used as a machine gun but also doubles as a chainsaw weapon.
Lightning Farron (Final Fantasy) wields her Blazefire Saber, a type of gunblade, which can become either a sword or a machine gun.
The Switch Axe (Monster Hunter) can be used as an axe but can become a sword as well.
Nanoha Takamachi (Magical Girl Lyrical Nanoha) wields Raising Heart, a staff that can become a blaster or spear.
Isabella “Ivy” Valentine (Soulcalibur) carries Valentine, a sword that can become a whip.
The Blade Blaster (Mighty Morphin Power Rangers) is a blade that also doubles as a laser gun as well.
The Mega Buster (Mega Man) can function as a Rocket Launcher, Flamethrower, Freeze Ray, Time Manipulation Device, and more.
The Sternguard Veteran Bolter (Warhammer 40,000) has multiple ammunition types including explosive, poison, armor-piercing, and super sonic.