In the Storyline mission Showdown in the Canyon, the final boss is holding a Flak Cannon, the player will obtain a damaged Flak Cannon after that, which couldn't be used because the shoot button will not be there in this mission on.
Like the Crucificator, the player can kill himself/herself in both Single Player and Multiplayer due to the fact it does splash damage. The same goes for all explosive weapons.
It sounds like the Crucificator when fired.
Flak Cannon ammo box has "Navy Shells" written on it, suggesting it is a modified shipboard weapon.
It may be a minimized and modified pack howitzer, as howitzers are also found on shipboards, so it may be plausible.
The term "Flak" is an abbreviation of the German word "Flugzeugabwehrkanone" or "Flugabwehrkanone", which translates to as "anti-aircraft cannon". This may mean the Flak Cannon is in fact based on World War II era German Flak guns, most notably the 88mm Flak 18/36/37/41.
When the player fires this weapon in Single Player, the ground will shake a bit.
Using this weapon over obstacles is not recommended as the player will take damage, but it will not show the stun affect.
It is one of the four Gold Weapons.
In Multiplayer, if you reload this weapon while standing or running, it will have custom reload, and if you reload while moving slowly, its reload will be same of a rifle.
Golden Flak Cannon has a firing sound similar to the Crucificator in both Single Player and Multiplayer.
This weapon is commonly compared to the Witch Impaler, but while the Flak Cannon arcs its shots, the Witch Impaler fires straight.
Among all its variants the Golden Flak Cannon does the most damage. The Fire Flak Cannon and Silver Flak Cannon follow it with each having about the same damage and the (ordinary) Flak Cannon having the least damage.
In coins, the damage of the Flak Cannons equate their price in which the Golden Flak as having the most damage is the most expensive at 170,000 coins, the Fire Flak Cannon and Silver Flak Cannon at 150,000 and 160,000 coins respectively and the Flak Cannon which has the least damage is had at 120,000 coins.
In version 2.8.2 the Golden Flak Cannon has been buffed making it now the most powerful variant, while the Fire Flak Cannon has been nerfed now equaling the Silver Flak Cannon's damage while the original Flak Cannon still does the least damage.
That version also nerfs the original Flak Cannon's fire trajectory although when fully upgraded all variants indicate no change in the statistic bars except in damage.
This weapon is extremely useful gun for long range splash attack damages, especially in El Matadero: you can go to the bridge and fire your ammo to kill or go to the center of El Matadero near the fountain and shoot. Either way is good.
The nerf of the Flak Cannon has made it very uncommon in Multiplayer, in favor for other weapons such as the Witch Impaler.
On the Multiplayer weapons console the Silver Flak Cannon appears as The Silver Consecrator.
There is a glitch where the player gets to buy the Golden Flak Cannon without even unlocking it. He/she will need to find an opponent in Multiplayer who have Golden Flak Cannon and let him kill him/her, and when the player dies on the bottom left hand corner shows the Golden Flak Cannon and it has no lock so he/she can buy it for 100 Sheriff Stars or even for 170,000 coins. This glitch may have been fixed.
The player can get the Silver Flak Cannon by getting killed in Multiplayer by it. Then it will have no lock and he/she can buy it for 160,000 coins.
The player can get the Fire Flak Cannon by getting killed in Multiplayer by it. Then it will have no lock and he/she can buy it for 180,000 coins. This glitch is already fixed.
There is a glitch on the Single player weapons console wherein Silver Flak Cannon appears as a normal Flak Cannon.