When you said the fps is 450, is that with the .43s or with .20s?
EDIT: Actually I think I may have an idea as to why it died. If your FPS is indeed with the .43s. then perhaps this occurred. With what you said the bucking seemed like it was tapered in the front then perhaps this caused a pinch jam, which resulted in a blow out of your patch and bucking. With the pressure being so high it forced the bb out of the jam. Which as you say the bucking is tapered down slightly into the barrel in the front makes sense. This theory would be a direct cause of a botched install.
Keep in mind this is Plazmaburn theory. I have been known to be horribly wrong from time to time.