"We've made a huge mistake"

Alternatively: when you pull the trigger, the gun disintegrates in your hands

Crawl: Stone Soup sort of has this with Banishment. If the target doesn't resist it's sent to the abyss. Cool. Not s problem anymore. Unless you later go (or are sent) to the abyss yourself, in which case everything you've sent could be there waiting for you.

Not all at once, have them start coming back one at a time as time goes on

(A) with items and powers they gained in the meantime wherever they got sent to
(B) and team up to fuck up the party, in squads of increasing power level since the party has been meeting increasingly powerful enemies of course
(C) the same, but changed a little bit in unsettling subtle ways that get less and less subtle and more and more wild and empowering as the story continues, like Pet Semetary

Pick one. Start the first little challenge they run into right after they disintegrate some massively powerful BBEG, so the realization of what they are in for now and the anticipation of what horror is in store at the end of the arc can build and build and build

Caution, they may say “fuck it” and still keep disintegrating stuff, or experiment with disintegrating each other once they know there are strengthening effects, things like that, so be prepared for that

I should do that with banishment, my players used it to trivialise every combat encounter with multiple enemies. Instead of banishing to another plane, it could temporarily send that enemy into the future, would make for an interesting fight!

Only works if they somehow reappear where the PCs are though, the enemy reappearing in the future in the same location wouldn't change a thing and it is locked to the gun then it's useless if they lose it/get rid of it...

Isnt that basically the entire plot of the loki tv show?

Plot twist: The entire world is destroyed in the future, so there's no real goal, except maybe kill all the other players and just hope there's a woman around somewhere to reproduce...