Hey guys, I'm trying to make a Spider Mine (like in Operation: Anchorage), since all the files are in Old World Blues, which is pretty awesome.

Anyway, I want the player to be able to craft and place them him/herself, so I'm using an ordinary landmine, which when placed on the ground, I want to be replaced with the Spider Mine creature, which would then scamper off and do its thing.

The thing is, I'm not really sure how I'd go about scripting this. I figure it'd be as easy as disabling the mine and using a placeatme/moveto command to replace it with the robot, but I'm not sure which blocktype to use. I don't think "begin gamemode" would work, but what about "onhit"? Would that be right?

You don't. Projectiles can't be scripted, you can only script a weapon or ammo with stuff that runs when it hits a target. I've got a working spidermine-style thing in Playing with Firepower (Dinky the Dyna-Saur) that's handled by having the projectile explode and using the explosion's Placed Impact Item field to drop the mobile bit.