# Penetration of a bullet into a wall

In which of the following situation will the bullet of the pistol penetrate deeper in the wall? Why? Alternatively will the penetration in both cases be the same? Can you explain the concepts behind this?

Maximum penetration will be achieved if there is a small (on the order of several centimeter) gap between the muzzle and wall. This is because, with muzzle against wall, the wall impedes the escape of air and combustion gases. Somewhat analogous to a muffler lowering the effective horsepower of an engine.

Beyond a "small gap for exhaust gases to escape," bullet velocity -- and thus energy -- and thus penetration depth will decrease with distance, as the bullet expends more of its energy pushing through the air on its way to the wall.

Would be interesting to do the experiment and find out where the maximum is. My guess would be around 5cm.

• What about the pressure from the gunpowder? If the barrel is touching the wall, then I would think that the pressure would continue to push the bullet since the only place to expand is still behind the bullet.
– LDC3
Aug 28 '14 at 1:03
• can u plz explain on what basis r u explaining this ? i think u r explaining it logically .... Thanks but is there any specific concept behind this ? Aug 28 '14 at 7:23

Pressure from combustion of gunpowder will far exceed pressure of air trapped when muzzle is touching the wall. The bullet will still be driven into the wall, but not quite as far, since it will have had to expend a small amount of energy against the trapped air ahead of it.

Not sure whether you were asking about my credentials or fundamental concepts. At any rate: I hold a BS and MS in physics and enjoy spending time on the pistol range when I get a chance.

HTH, Ken

• Depending on the size of the gun itself, I would guess that holding a gun to the wall tightly would be difficult to do. I would not advise someone actually trying that with a 45 caliber pistol for instance. It is likely to be kicked back into your face. Nov 22 '14 at 4:02