I was watching a TV show where a criminal used a bullet made out of blood, so that after entering the body the "evidence" (the bullet) melted away. I was wondering if it were possible to do something like that. That is, is it possible to make a bullet out of blood, ice, or a similar meltable substance that could be fired without melting and that would be capable of piercing a target and that would subsequently melt away? Any supporting physics/chemistry equations would be appreciated. Thanks in advance!
The reasons you are looking for (why it doesn't work) are twofold:
First, mass. Bullets are made out of lead, tungsten, depleted uranium etc. Heavy stuff. Aluminum and brass would make perfectly good ammunition if they weren't so light (or if the shootout was on the moon). The problem is lighter projectiles are greatly affected by air resistance and lose much of their accuracy and deliverable energy enroute to the target. The bullet is not so useful if it gets blown off course, or simply bounces off the target. Test this yourself - get a baseball, a baseball-sized ball of styrofoam, and a baseball-sized ball of lead. Have someone toss them from a 3rd-floor balcony and catch them bare-handed.
Second, heat. All that gunpowder igniting behind the bullet gets rather warm. And there's friction with the inside of the barrel. Plot that heat against the energy needed to boil 6 grams of water and we find that our ice projectile melts in the barrel, not in the target. And if it doesn't melt completely it certainly won't melt symmetrically, and a mis-shapen blob won't go where we want it to go.
Tricky with a typical explosive propelled bullet but if you have a compressed air gun handy then it's quite easy. It is even the subject of a long lived Urban Legend
It's not feasible, this was tested by the MythBusters (assuming that a frozen blood bullet would be similar to an ice/meat one):