You can get the weapons from a rampage by failing it in a replay. What you need to do is simple: trigger a replay the exact frame the rampage ends. It gives you rockets for Silence the Sneak, E2G, Bait, Arms Shortage, etc, if you pull this trick off with the Rocket Rampage during Payday for Ray.

The optimal way of doing this is to spam a replay from another island, as the loading time seems to allow you to "buffer" the replays to some extent.

making a short replay and playing that is very consistent for me as well. we'll need to time the rampage's end right, though, since it can't really be in the cutscene

Is this a new idea? F2, start with F3, end with F1 so that you can start it with F3 immediately after. In VC, you can end F1 replays with F3, in III that only works with F3 replays...

This trick is frame perfect, cancelling the replay costs you at least 1 frame. It's bad. If you have a replay from another island so it starts with a load, you can spam it pretty much every frame in a very quick succession, so it's consistent.