There's just one catch to Paul's.....It's for the Amiga Computer. Unless you have an Amiga or an emulator, you can't use it. I got an emulator for it and a copy from Paul, and it's great! Really cool. I would recommend WinUAE. It's an Amiga emulator for Windows, and pretty easy to figure out how to use it. Just search for it on Google.
Somebody also made a program for the GRiDCase 3, but I don't know too much about it. Then, I heard awhile back, that somebody was working on a flash version, but haven't heard anything lately. Hopefully Tony will archive (if he still can) the sentry gun section on the old board. If he archives it, that will answer most of your questions. So Tony, if your reading, please try to archive the sentry section.
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M-40 grenade: $30
Pulse Rifle: $2000
Tactical Smart Missles: $10,000
Sulaco Full of Badass Colonial Marines: $700,000,000
Kicking some alien ass: Priceless