Stradwolb
Description: they look like normal blow darts, although the faint blue glow is a little disconcerting if you have a background in nuclear physics. Yup, Cherenkov radiation! But don’t worry, that’s just a reaction from all the tachyons. There’s a small hollow section that can be used to fit a small note in.
There’s a lot of double talk about time travel and quantum entanglement and closed temporal loops involved, but the process itself is simple: you insert a stradwolb in its specially-made blowpipe, and shoot it at someone or something. When it hits, it uses up all of its stored energy to travel back in time for a variable amount; the average human being, if he blows really, really hard, can send stradwolb back about five minutes. It will reappear loosely attached to whatever or whoever it was shot at (not enough to cause damage, but enough to not immediately fall off, either).
This causes fewer problems than one might expect. The only meaningful temporal paradox that gets generated is from the existence of the stradwolb itself, and the universe handles that by vanishing the item permanently when it hits its original ‘present.’ Otherwise, just assume a new timeline was created. That happens all the time.
This is not an effective weapon, in other words. It’s more often used as a retroactive alarm system. Get ‘shot’ with one, and that’s a pretty good sign that future-you wants present-you not to be surprised at whatever happens next (or before, depending on how you look at it). Not having a note inside the stradwolb is even worse: it means that whatever happened to you went so badly so quickly that nobody had time to explain the situation. Better ready those weapons then, hey?
And watch the ceiling. Nobody ever looks up. That has nothing to do with stradwolb; it’s just a general observation about ambushes.