The Blunderbuss was typically viewed as a small, muzzle-loading shotgun. Its defining feature is the flared barrel, which can come out anywhere from three centimeters to three inches in diameter. The flare was not used to spread shot (a common misconception), but instead to make funneling of gunpowder easier whilst riding on horseback or stagecoach, where it was intended to be used.
In game, the gun uses pieces of zombies (ribs, eyes, ears, and tongues) collected from slain undead, in place of traditional ammunition. When fired, the blunderbuss explodes zombies into pieces. It mutilates the corpses so horribly, Marston is unable to loot anything from them.
The Blunderbuss can have a maximum of 19 spare rounds and one loaded and it takes 10 of any undead body part to create one round. After the player completes the Undead Treasure Hunter Challenges, they will receive extra room for much more ammo for all weapon types, including the Blunderbuss.
The Blunderbuss can destroy several zombies at once, as long as they are either arranged in a straight line or closely bunched together.
When reloading, the player travels at a walking pace, unable to run, and will not change weapons until the reloading is complete, so escaping from the Undead becomes much more difficult and shots should be planned accordingly.
While reloading, Marston can still rifle butt his enemies or roll away at a slightly faster speed than walking. Sometimes when the player rifle butts an enemy while reloading, it will initiate an execution animation, sticking the blunderbuss under the chin of the enemy and pulling the trigger. It will not use up any ammo and will not vaporize the target. Which will result in the same results of a lethal rifle butt (provided the player is still in the process of reloading).
The Blunderbuss reloading animation can be skipped by holstering the Blunderbuss, then re-equipping it.
The Blunderbuss can also be blind-fired behind cover.
In the teaser trailer for the Undead Nightmare downloadable content, John Marston is seen holding the Blunderbuss with two hands, as it is intended to be used. Contrarywise, the promotional art for the DLC shows a Marston holding it with one hand, not unlike the Sawed-Off Shotgun.
This is likely because the DLC's promotional art was designed around the original, and redrawing an additional arm onto the gun would break the similarity.
It is also possible that the promotional art gun is a blunderbuss pistol, or dragoon, which is a shorter variant meant to be held with one hand.
The Blunderbuss' presence in Redemption is altogether extremely odd, as the gun was discarded commercially in the mid 1800s, well before the events of the game, as commented by Marston when the gun is given to him by West Dickens.
This is true in the storyline in that John could not have bought it anywhere, but traded scrap metal parts for it, though at first did not know what he was receiving in return. A rather disgruntled John points out the gun's antiquity, exclaiming "A Blunderbuss? What is this, 1850?" This shows that as the scarcity of weapons and conventional ammunition increases, trading of the former and latter for services is becoming the common economic system of trade. This also shows in the fact that the Blunderbuss, as West Dickens said, can be loaded with just about anything, removing the need for conventional ammunition and so improving its usefulness despite its age.
While actually firing objects not intended for use as ammunition from a blunderbuss could damage the weapon, there is some truth in Marston's use of undead parts as ammunition- there are accounts of improvised ammunition such as nails or even rocks being used as ammunition in blunderbusses.
Blunderbuss comes from the Dutch word 'donderbus" which means "thunder gun" or "thunder pipe".
The Blunderbuss acts very similar to the Explosive Rifle from the Liars and CheatsDLC as it completely pulverizes zombies that are hit, turning them into a fine red mist and leaving a red streak where they were standing.
The reloading animation of John packing down the Undead body parts along with the gunpowder with the ramrod stays true to the same way a real Blunderbuss is reloaded, except that he does not prime the flash pan with powder, something that would need to be done in order to make the weapon fire.
Because it is a muzzle loader, the Blunderbuss requires a black-powder charge to propel the shot; however, Marston never loots black powder from dead zombies or out of ammo chests, but somehow he always manages to never run out of it.
The Blunderbuss, when shot at humans, functions like an ordinary shotgun, not blowing up the hit target, but causes a large amount of damage or instantly kills, even at further ranges and with shots landing on limbs.
When playing Undead Overrun, a glitch can occur where a Bruiser takes hits from it but will keep coming as if it were just as powerful as any normal bullet.
Sometimes after the player gets the weapon, they will be able to keep making ammo even if they don't have any Undead body parts.
When returning Missing Persons, the Blunderbuss will not vaporize zombies but will behave like a normal shotgun. It will still leave a red blood streak; however, the corpses will also be able to be looted.
Oddly, when reloading the Blunderbuss while running on horseback, John Marston makes a bizarre animation of reloading the weapon with one hand.
Commonly, when executing an Undead with the Blunderbuss, the player will reload the gun without the reload animation playing.
Being close to an Undead animal (for example an Undead Bear, Cougar etc.) may sometimes cause a glitch in which the Blunderbuss behaves like a normal shotgun, doing almost no damage to the Undead animal. This has been repeatedly reported while on horseback.
While on horseback, the Blunderbuss will sometimes clip through John Marston's hand.
Sometimes, but rarely, if you reload while charging at full speed with the speed glitch on Famine, then Marston will have his arm stuck through his back (it works with any horse but Famine is the best).