Staking the grenade down still needs a means for the tripwire to break free from the grenade. Perhaps a thread pulls the grenade out and snaps when the grenade pulls against the stake. There are many ways to skin this cat really. Whatever you do, it probably would be best to set up your trap so the grenade doesn't go rocketing off into the bushes (the GSG-1 kind of jumps around when it goes off) and the tripwire also shouldn't yank the grenade far from the setup point so you need the tripwire to pull free once it's done it's job.
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