So i went to go get what I suppose is the final gun upgrade, but I ran past it and triggered the cutscene where you have to run away from the explosions, so now I can't back to where the gun is. Is there some way to get it or am I just fucked and going to have to start over to get 100% items?

i did the same thing, and as far as i know, you can't go back. then again, i can't work out how to find it BEFORE the room's exploded. all i can get to is a vent that's too tall to double jump up

You can't go back. Same thing happened to me.

The only possible way to get the final gun would be to unlock the secret room by collecting every gold bars.

" i did the same thing, and as far as i know, you can't go back. then again, i can't work out how to find it BEFORE the room's exploded. all i can get to is a vent that's too tall to double jump up "