Theoretical maximum high scores

[ManOkuChou]

Is it known what the theoretical maximum high scores, with perfect play, for various shmups are?
Specifically, I am interested in Ketsui. The scoreboard in Ketsui appears to have 10 digits: why is this? Is a score of 1 billion even possible with perfect play?

[guigui]

I guess computing highest max score for many manic shmups is impossible, especially the ones that include some sort of Hyper time that boosts score. Ketsui included in this category.

Old school shmups it may be easier to compute, though it may be easier to just make a TAS superplay and see what score comes out.

[BareKnuckleRoo]

Games generally have more digits available than can be realistically obtained, so as to avoid the potential of a counterstop (where the score's so high it can't be displayed beyond all available digits showing 9s).

[el_rika]

With Cave, you can never know.
There's always some glitch/feature that can skyrocket the score.

[OmegaFlareX]

See STG Wiki Hall of Records and this thread which collects the highest scores that have been found put to video.