Could a mini-crossbow work as a deadly weapon?

Yes, it could be, and one hand crossbows have been made. They were never a common item, but they were effective short range weapons. The power would be more a matter of the spring (bow) than anything else.

You're counting on puncturing someone with a small, sharp object. Think perhaps four to six inches long. THAT'S your question. Sure, people have been stabbed with knives and killed by them - so its PLAUSIBLE to shoot someone with a 4-6inch long quarrel (Fins will prevent some penetration) - but KILLING them as in 'drop dead' killing them? A VERY lucky heart shot might do it - but not likely. Most likely piss them off - and have them come at you.

As a threatening device? Sure - make your enemy a coward.

Here's a link to a mini (very mini) cross bow that folds up and is easily concealed and fires a six inch quarrel (or bolt the "arrow")

http://www.youtube.com/watch?v=EBn7NJg6grw

Here's a link to an 80 lb mini crossbow that is more than adequate for short range hunting. It demostrates the power of the weapon. Good video, poor audio, but notice the penetration power of the arrows:

http://www.youtube.com/watch?v=PcGALLy_V-s

Hope it helps

If you shoot the bow and miss, you can always throw the weapon at your target.

StephanieFox said:

If you shoot the bow and miss, you can always throw the weapon at your target.

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Or tie a bowie knife to it and use it as a bayonet.

Maybe the bolts could be coated in a powerful venom substance?

As a offensive weapon, you would have one shot, so element of surprise might work for an assassin.

As a defensive weapon, again, one shot, but it might make your opponant pause, thus giving you time to make good an escape or go for a more defensive weapon.

I don't see this as a main weapon...

Kaiser-Kun said:

Or tie a bowie knife to it and use it as a bayonet.

Maybe the bolts could be coated in a powerful venom substance?

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This.

Or a little explosive charge planted inside the tip.