The shotgun, better known for its nickname 'boomstick', is a weapon that Ash uses in The Evil Dead series and appeared in The Evil Dead, Evil Dead II, Army of Darkness, including the comics and video games.
Ash himself explains many of it's features in Army of Darkness: "The twelve-gauge double-barreled Remington, S-Mart's top of the line. You can find this in the sporting goods department. That's right, this sweet baby was made in Grand Rapids, Michigan. Retails for about a hundred and nine, ninety five. It's got a walnut stock, cobalt blue steel, and a hair trigger."
The weapon is a constant companion for Ash in any incarnation of the Evil Dead franchise. In the franchise, it's most commonly seen dressed similar to his appearance in Army of Darkness, in a handy leather harness strapped to Ash's back.
In the games, the 'Boomstick' is often the first weapon Ash starts with and is upgraded as the games go on. Besides the sawed-off Remington shotgun, Ash is also skilled in the use of other firearms, like the Winchester Model 1892 lever-action rifle which he used at the end of Army of Darkness.
The 'boomstick' first appeared in The Evil Dead, when Scotty and Ash investigate the cellar in the cabin. However, it was completely different in style and design. It was originally a single-shot twenty gauge shotgun with no modifications. (The yellow shells tell you it is a twenty gauge) It's used generally throughout the film.
The 'boomstick's ' second appearance is in Evil Dead II, and this is when the shotgun begins its evolution into the 'boomstick'. Ash first uses it after he uses the chainsaw to saw Linda's severed head in half in the workshed. Many notable things occured with it, such as Ash shooting his severed hand through the wall and Ash shooting Bobby Jo through the door. However, the most notable moment with it is, once Ash recieves his chainsaw-hand, he saws off the barrel and creates a sawed-off one, creating the famous 'boomstick'. Instead of a single-shot, it's now a Remington 12-gauge double-barrel shotgun, and now includes a leather harness that Ash wears that he puts it in.
The 'boomstick's' third appearance in the Evil Dead franchise is in Army of Darkness, and this is the only time in the series Ash calls the shotgun his 'boomstick'. When Ash is captured by knights, it's confiscated, as is his chainsaw. He breaks free, however, and retrieves both. Once Arthur confronts him with a sword, Ash blows the tip of the sword clean off and gives a long speech about his 'boomstick' and S-Mart, before shooting a deadite into the Pit. For the rest of the film, Ash uses it commonly, most notably when he shoots Bad Ash, his evil alter-ego, in the face. The 'boomstick' pretty much looks the same from Evil Dead II, except being much dirtier and, for some reason, the barrel isn't as cut down as it was originally.
The different Boomstick (which did not belong to Ash) is found by David, and Eric in the Fruit Cellar, along with the Necronomicon. Mia is the first person to use it (shooting David in the arm). It is eventually used by David to shoot off the possessed Natalie's arm off and to burn down the cabin by shooting a can of gasoline, where it presumably is incinerated.