The Tack Shooter requires an Iron Block at the center of the crafting recipe, and darts all around it. It will appear gray, but you can use dyes to dye it. When placed down, it will have 30 HP (15 hearts). When bloons get within range of the tack shooter, the shooter will attack with a dart (see Dart Gun) in all adjacent blocks. Unlike the Dart Gun, this doesn't require ammo. It can be mined instantly by you, but it is as hard as diamond to any bloons able to mine.
Faster/Even Faster Shooting ($210/$300, exactly what it says on the tin.)
Tack Sprayer ($500, shoots 2 darts per block.)
Ring of Fire ($2500, every block within range is hit with 5-10 fire damage and set on fire, the tack shooter becomes immune to fire, and it gains an extra block of range for good measure.)
More/Even More Range ($100/$230, increase range by 1 block each.)
Blade Shooter ($680, shoots blades that do 5-10 damage and deal 1-2 damage to adjacent bloons.)
Blade Maelstrom ($2700. Once per 30 seconds, activate this to shoot 100 blades in all directions, with a range of 20 blocks.)