So I opened up my stampede to clean it before a weeklong, and found what appears to be corrosion(or at least discoloration) on all the heads of the internal screws.
I'll bet that 99.99% of the problem here is encapsulated in the tenth word.
Taping up seams and openings while out in the rain sounds like a great idea. Only a few drops will get through. But after the game, those few drops can't get out. For weeks. You have what some of us call "Super Soaker Rot." If you've been out in any wet conditions with your blaster, then open it, clean it, and dry it, as soon after the game as you are able. At the very least, remove any and all waterproofing measures as soon as they're not actively helping, because you can count on them working against you in storage.