I had a Point Blank pcb and the Namco guns had there own Gun pcbs. I also owned Lethal Enforcers and the guns pluged directly into the jamma pcb, much simpler.
You find many hidden secrets/rooms, find the secret weapons? Did you actually finished the game by getting all three crystals? What crystal did you find the hardest to get?
I think I may have found one hidden room, but I don't recall many crystals. I usually play them at auctions I have attended and I have never really tried to beat it. Mainly, because I really want one and a game looses something once you beat it. I have yet to beat some of my favorite games. But Zombie Raid is still my favorite in the genre.
On a side note, Space gun is worth playing for the game of space invaders(via the gun) at the end of it. Pretty cool.
Another "space is no object" game is LA Machineguns. Huge 50 something inch screen with a sub in the metal floor. When the enemies hit the ground, you feel it in the floor.