What happens if you strike the percussion cap / primer on a bullet?

Unlikely that you'll ever set off a cartridge like this... I only know of one time this has happened. A guy threw a shotgun shell into the seat of a pickup with a screwdriver in the seat. Screwdriver tip managed to set off the primer.... It's a one in a million occurrence.

What will happen is the bullet will fly out of the casing at a harmlessly low velocity. Maybe if you were struck in the eye, it would do damage. I seriously doubt the actual bullet would penetrate skin. It would likely be in almost perfect condition... No rifling cuts, maybe scratched up depending on what it hit. It isn't going to be seriously deformed.

Your primary danger is from fragments of the casing which, although small, will achieve sufficient velocity to cause harm. Due to their small mass/density, they will still probably just barely penetrate the skin... You'd likely be picking them out with tweezers. Again, damage to the eyes would be the primary danger.

Bullet Primer

If the cartridge is accidentally fired IN THE OPEN, there likely will be very little damage.

The primer (the cap) likely would back out of the casing at a fairly high velocity, but it has such a low ballistic coefficient that it would not go very far, but it would go fast enough that if it hit you, you WOULD feel it. The actual BULLET (the projectile) might be thrown a few inches, but that would be all. What would be the greatest danger would be the brass casing, as it would open up into a pretty brass flower, scattering bits and pieces of brass all over the place at fairly high velocity. The primary hazard would be to the eyes, but ordinary eyeglasses are enough to protect you in this case.

Overall, the effect would be exactly the same as if you set a live cartridge off in the garbage. You can duplicate this, if you like, by putting a live round into a pot, putting the lid on very carefully, and heating it over a campfire until it explodes. The pot and lid trap all the high-velocity bits and pieces and you have an excellent conversation-piece for your cartridge collection.

But if the cartridge is CONFINED, then terrible things can happen. There was a recent case of an American soldier in Afghanistan who used a live .50-calibre round to bang in the mounting-pins for his Browning .50 HBM2 machine-gun. The cartridge exploded as he was holding it tightly. The photographs of what is left of his right hand are enough to cost you your dinner, your lunch and your breakfast, all in one massive chunder. The poor man is crippled for LIFE, all because he didn't THINK for one second.

NOT something to play with. Ammunition, like firearms, should always be treated with respect. If you don't understand them, leave them alone and DON'T disturb anything.

Just common sense, but sometimes it's not common enough.

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I've seen rounds detonated from heat but never from something striking the primer. A tightly crimped bullet won't even leave the case mouth when this happens rather the case ruptures and launches the entire cartridge.

All of the cases I saw were brass so they didn't shatter and send small pieces of metal flying (steel cases might, but I don't intend to check). The cartridges that burst did fly off in random directions and from the sound of them whistling through the air, they had a pretty high initial velocity but being ruptured and bent out of shape they were not very aerodynamic and lost speed quickly. I think if the case hit you at close range there would definitely be a bruise and maybe some scratches from the jagged edges of the ruptured case, but unless they hit you in the eye, I don't think they would cause any real injury.

It would be extremely hard to hit the primer with enough force to set off the cartridge, because often an old gun with a worn or broken firing pin will indent, but not fire, the primer.

But in the rare possibility that you hit it just right, the brass shell would explode, and the bullet wouldn't go hardly anywhere. I saw a video once of a box full of ammo that was set on fire. The box burned, the primers and gunpowder popped like firecrackers, and none of the debris went more than a couple of feet.

A soldier recently blew most of his hand off when he used a .50 BMG round to hammer in a pin on the M2 machine gun which set off the primer. Assuming the round did go off by hitting the cartridge on a pin or nail sitting on the desk, it could do some damage to the person's hand. The bullet itself wouldn't do much damage since it gets its energy from the expanding gasses in the barrel. Without a barrel there isn't any way the gas could accelerate the bullet. The powder would probably flare up right in front of the person's face and cause burns.

Percussion Cap

I have only heard of this happening at a range once the guy had left a box of 45 acp ammunition lying open on the bench and lo and behold the case fired out of the pistol landed just right on the primer the rim hit the with sufficient force to set off the primer, the box of ammunition that was in the box was spread all around and the styrofoam packing was floating in the air he only foiund out after picking up the cases and noticed the indent on the primer

The case will split. The primer will pop out of the case, and being light, may fly a few feet. The bullet, being heavy, will not go far.

if you set off the primer of a round that is unconfined in the chamber of a gun, the bullet will go one way at a decent speed, but the casing will go in the opposite direction at velocities high enough to penetrate your skull.