Senbon are a readily available tool to shinobi, however the skillset required to use them effectively is far from basic. Sharing more versitility than their kunai and shuriken bretheren, the senbon may be used as part of medical apparatus for surgery, acupuncture and more. They are also a useful tool for interrogation as their high penetration rate and common psychological phobia surrounding them.
Due to their size and streamlined structure, the user would find great difficulty placing an explosive tag upon a senbon. As a lone weapon, the needle may be considered a mere nuisance in the company of a heavily armoured foe or an untrained user. However, the relatively light weight and compactness of the needle allows the user to conceal and carry maginitudes more of them in comparison to other projectile weapons, even rivaling the smallest of shuriken. Poison may also be applied to needles with ease, allowing the concoction to infiltrate the target's bloodstream directly.
Overall, the senbon has two main roles in battle. One is to conduct a precise and delibating strike, hitting either key nerve clusters in order to impair movement or arterial pathways and exposed organ tissue for mortal, if not fatal, damage. The other is to provide an overwhelming screen or wave of needles to discourage or intimidate the enemy through sheer numbers.
A senbon is simply a thin and long cylindrical needle, created most prominently from substances such as iron or steel, with an average length of 6 inches (15cm). Both points are sharply tipped and variants such as ring-tipped akin to a kunai's handle are uncommon if not rare due to the weapon's nature of mass production.
The sound of a senbon is next to nill. Combined with it's acute point and almost indiscernible presence in flight, it is the pinnacle of stealth takedown weaponry.
The origin of this tool for a shinobi is up to discussion, yet due to the ease of concealment and various applications it is no suprise that they are utilised by skilled assassins.