It looks like an ordinary wakizashi. The hilt is light-blue with a tsuba made of two slim swirls of metal looping around the blade to make an open oval, resembling an "S".
Shikai: It is triggered by the phrase "Shoot to kill" (射殺せ, Ikorose). In its Shikai, Shinsō's blade glows white and extends at high speed to impale wielder's opponents from a distance. The blade also carries tremendous force when extending. In addition, wielder can maintain the extension and swing the activated Shinsō in wide arcs, attacking multiple targets quickly and simultaneously. Shinsō is capable of reaching a maximum length equivalent to one hundred times its original length, earning it the nickname Hyapponzashi (百本差し, Hundred-span).
Bankai: Kamishini no Yari (神殺鎗, God-Killing Spear): In its Bankai state, Shinsō does not change in appearance at all, retaining its small wakizashi form. However, as Kamishini no Yari, the blade is capable of reaching lengths equivalent to 13 kilometers (roughly 8.1 miles) at 500 times the speed of sound. The blade can also retract to its normal size at the same rate.
Bankai Special Ability: the Bankai's main basic abilities are essentially the same possessed by the Shikai, but its power, length and speed are vastly amplified. The blade's tremendous cutting power is significantly increased, to the point where wielder is capable of swiftly cutting an entire town in half with a single stroke while standing a great distance away.
Poison: Kamishini no Yari's true ability as well as its deadliest aspect is not based on its length or speed but rather its ability to turn into dust for just a second when it expands out and contracts. There is a deadly poison inside the blade that dissolves and breaks down cells. Wielder can leave a sliver of the Zanpakuto in an opponent when they retract Kamishini no Yari, allowing them to kill an opponent at any time they choose to by uttering the release command "Kill, Kamishini no Yari" (殺せ, 神殺鎗, Korose, Kamishini no Yari) for the technique, while placing their hand up to the target. The result causes the target to completely break down and dissolve at the cellular level from the inside out.
Blade Extension & Contraction: the Bankai can achieve its full length at 500 times the speed of sound: exactly 171,500 meters per second in order to arrive at its full length of 13km in under 0.08 seconds.
Butō (舞踏, Dance (Step)): By adopting a stance where he firmly grips Kamishini no Yari with both hands and places the Zanpakutō's hilt squarely to the center of his chest, wielder is able to unleash the tremendous extension and contraction properties of his Bankai to an even more terrifying extent than its normal state. This occurs in the form of a piercing maneuver where the extension and contraction of the blade becomes almost completely-unseen to even the most-diligent of observers.
Butō: Renjin (舞踏連刃, Dance (Step) Serial Blade): This ability repeats the action of extending and subsequently contracting Kamishini no Yari, as observed within the technique Butō, multiple times in rapid succession. The entirety of this process is completed in a mere moment, resulting in the individual steps involved in this technique becoming virtually indistinguishable from each other, generating an apparent mass of numerous blades that leaves little opportunity for the intended victim to evade successfully.