There are airsoft grenades out on the market, the new pfi one looks decent. And madbull is working on a one time use low cost grenade. Neither of those grenades will put someone in the hospital if they hit someone in the head. Your grenades dont sound safe at all, sorry, but they dont. If someone gets hit by one, judging from your discription of the grenade, they would probably be seriously hurt. Not to mention, 'shaped like a pipe bomb', that could cause some serious problems... And finally, sounds like you're having the same problem that seems to plauge the escort hand grenades, aka, an unreliable timer. All in all, yours is not a bad idea, but as a throwable hand grenade, not safe, as a mine or bomb simulator though, good idea.
Really, I just prefer to wait for the safe, cheap madbull disposable ones.
EDIT: Why not try redoing them as claymores? We could use a realitivly cheap, but effective claymore. The most effective claymores are so expensive that you cant afford to get more then one or two, and the cheaper ones have limited range and dont give you that nice "POP" that you get with gas.
Guns are meant to go bang, not bling!
Just say no to chrome guns.
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