Waffengeists are the result of not only the gun's experiences over it's service life but also it's users' experiences and feelings towards it over it's service life (this ties into a theory related ghosts and spirits, that things and places can store "energy" or memories. Yes, this does mean that other objects can function similarly.) As such theoretically any gun can have a waffengeist, but in practical terms this depends on it's service life and how strongly it's user/users felt about it; more on this later. Because of this older guns with a history of military service and/or users who have had strong feelings towards (note, feelings in general) or strong emotional events with (again, note, events in general) were more likely to have naturally strong geists than other weapons. For other waffengeists, it could be the result of being cherished by their owners, or being associated with a significant emotional event (near death experience, or perhaps a brutal murder.) For those wondering about things that are not necessarily guns, we will get to them later.

With the convergence and the magic and other nonsense it brought came the ability for waffengeists to be "awoken" or "activated" from their dormant states. Active/awake geists can materialize into a human looking form as well as manipulate their their gun form; dormant ones cannot, but post convergence they are still aware of their surroundings and can communicate with other geists. At the time of convergence, some strong geists automatically were awoken, but the majority need a ritual known to certain magical artisans of the fantasy side to awaken them. This ritual involves the inscription of a magical sigil or runes onto the weapon, which along with some magical material gives the geist enough energy to awaken. The amount of magical material required is inversely proportional to how strong the geist actually is, hence theoretically any weapon can be made to have a waffengeist but practically there is a point where the amount of material required to have adequate activation energy becomes impractical; any weapon needs to have had some form of a service life with a user in order to have a waffengeist, a new gun cannot magically just be made to have one.
All active geists have a sigil or set of runes on their gun form, usually the bolt (refered to as it's "mark"), or in the case of other weapons on whatever could be considered the "heart" of the weapon.

Strong Geists can control how they appear to others, but weaker waffengeists' appearance is influenced more by how the user imagines her; similarly their personalities can also be influenced in this way but the longer the service life of a waffengeist (including time after awakening) the more they will have an independently formed personality. 
While they think and act independently of their user, waffengeists exercise an innate subservience to their users and an urge to be as useful and effective a weapon as possible; this can be through being another pair of eyes looking out for threats, through occaisional self maintenance, through instruction on the best way for the user to use them, to even operating themselves (as an example, a bolt-action geist cycling her own bolt while the user aims and fires in order to allow the user to better track targets and deliver follow-up shots faster). While Waffengeists can do basic maintenance on themselves and manipulate themselves, they cannot fire themselves, ever; this seems to be an action that only the user can perform, even if the user orders a geist to fire themselves (though there is anecdotal evidence that in very extreme circumstances, usually when the user’s life is in danger, that they can fire themselves, in a few cases using their gun from in their human form.) It should also be noted that they do not need to manisfest into their human form to talk to the user.
Their willingness to do all this depends on their mood, personality and experience with the user; in effect the relationship a user has with their geist. They can sometimes form very deep bonds with their users, to the point of going dormant or even refusing to be used at all by anyone else if a cherished user dies.

Finally, waffengeists as a thing are very hard to kill. While damage to their gun form can and often will reflect in their human form (how so depends on what analogies a user would make between gun damage and human damage), only extreme damage to it's mark can make a geist dormant again, and only the complete destruction of it's gun form can "kill" a geist.

It is a theory that the avatars and angels of the Murder/k/ube (part of our canon) are in part the detached and vengeful souls of the waffengeists of the weapons slagged for the "artpiece."

Notes on this lore doc - “created” or rather pastebin created September 24th year of our lord two-thousand and twenty.