Sounds like it will still be too heavy/hard. You'll need at least some metal parts to handle the pressure of the gas, over the long term, plastic wont cut it. And the weight of the CO2 cartridge alone is too much. PVC piping is pretty much the only easy to obtain, cheap to use plastic that i can think of that will handle the pressure of CO2, and it's too heavy and solid to be safe to throw at someone. Even a single pound of weight is too much to safely throw in someones direction. This is why madbull decided against using a gas to power their hand grenade design. Any gas system will be either too heavy, or wont last in the long term.
Hockey tape wont provide protection from injury either.
You need to consider the amount of force someone is likely to throw it with in the 'heat' of battle. A plastic pipe full of powder to the head could still easily cause damage.
Guns are meant to go bang, not bling!
Just say no to chrome guns.
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