Despite the ability to blow a Titan's head off in one shot, half of all Titans will merely regenerate their heads; the only certain way to kill a Titan is to destroy the nape of its neck. In addition, the time it takes to reload and aim a cannon makes it inapplicable for offensive strategies against Titans. However, cannons are still used in static defense positions, most notably on top the Walls and near gates in the event of a breach. As such, they are used most often by the Garrison Regiment, who patrol such areas.
Outside of Paradis, the developed militaries around the world make use of various advanced types of artillery pieces which fire modern artillery shells rather than primitive round shots. It is also more mobile than its Paradis counterpart since it is ubiquitously wheeled, with some variants mounted in armored vehicles like battleships or armored trains. Notable examples include heavy mortars and the Anti-Titan Artillery fielded by the Mid-East Allied Forces.
In Paradis, the cannons fire two different types of ammunition: High-Explosive Shells and Grapeshots. Given their poor accuracy, the cannon barrels are most likely smoothbore. The ammunition is breech-loaded, fired by pulling a string connected to a firing pin, and it takes about twenty seconds to reload.
High Explosive shells are used to attempt to kill a Titan by destroying the nape of the neck in the blast, but the cannons have terrible accuracy, making these shells almost useless at longer ranges.
Grapeshots are the more traditional cannon rounds and are mostly used to slow down Titan advances with their wide, dispersion shots (which are designed to take off as many limbs as possible in a wide area).
Armor Piercing Shell: this type of 100mm ammunition is strong enough to pierce through the nape of any Titan, including one of the Nine Titans, without any difficulty. The technology used to create piercing shells is only known to the Middle Eastern Alliance with their Anti-Titan Cannons.
Howitzer Shell: this type of ammunition is used for long-range artillery barrage, but is ineffective against the Armored Titan. The Marleyan military often uses this type of cannon to destroy enemies at far distance. At some point between the year 833 and 845, heavy mortar crews belonging to an unnamed opponent of Marley fired a volley of explosive shells upon the Armored Titan to little effect.
Prior to the breach of Wall Maria, Wall-mounted cannons had limited mobility and could only fire from one point inside or outside the Wall. After said breach, the Garrison was forced to bring them on the ground as to repel the Titan advance further.
Later developments led to railroads encircling the entire Wall, allowing cannons to bunch together and focus fire on a single point if necessary. Before firing, the cannon must be solidly fixed to prevent the recoil from blowing it off the tracks. Wall-mounted cannons also possess the ability to fire directly downward, which can allow them to release punishing volleys of fire at Titans with no threat of reprisals.
Cannons were used primarily by the Garrison after the fall of Wall Maria to repel the incoming Titans. The cannons placed on the ground, however, had little impact due to low range weapons targeted at high altitudes. The Garrison used these cannons to guard the inner gate of Shiganshina, but they could not stop the Armored Titan.
Eren Jaeger, among the other cadets, were assigned to clean the cannons whilst awaiting further instructions. After the breach of Trost District, the Colossal Titan deliberately disabled the cannons and the railroads on top of the Wall to prevent reserve cannons from being brought in. One of these cannons was used in an attempt to kill Eren, Mikasa, and Armin, but Eren stopped the cannonball by shifting and developing a semi-transformed Titan. After sealing the Trost gate, wall mounted cannons fired highly explosive shots, killing the Titans swarming near the Wall. This continued for a whole day.
Whilst under assault inside Utgard Castle, Scout Regiment recruits Reiner, Bertholdt, Ymir, Historia, and Conny begin gathering supplies to stop the advance of the small Titans within the castle. To stop one Titan, a cannon is rammed into it, stopping it from moving.
The Garrison in the Orvud District used concentrated cannon fire against Rod Reiss's Titan form in order to kill it. Although the cannons greatly damaged its back, they were not enough to stop its advance or to blow off the nape of its neck.
- The fact that these cannons are quite inaccurate may come from their lack of rifling. Rifling is the process of making helical grooves inside the barrel of a gun. When a projectile is fired, the grooves make the ordnance spin, improving its aerodynamic stability and increasing reach and accuracy.