OK, that is a GREAT question that also causes a lot of confusion and continues the myth of powder burning the entire length of the barrel. So let's see if we can put that one to bed as well.
To explain this one, we have to use chemistry, the gas laws and basic elements of physics.
The powder burning completely in the barrel and in only a couple inches or so from the breech, causes gas pressure in the barrel. It is the gas pressure that forces the ball/projectile/shot down the barrel.
I'm not going to bore forum members to tears and causing them to nod off to sleep before the explanation of the gas laws is done, though. I think most people understand that gas pressure inside a container pushes equally against all sides or edges of the container. A common example of this, though, is when you add air pressure inside your vehicle's tires, that keeps the tires inflated equally all around in the rim with serviceable tires.
OK, so the powder is burnt entirely in only a couple inches from the breech, that raises gas pressure incredibly inside the barrel. That incredible pressure acts against against the patched ball or shot and out the vent hole, with equal pressure. However, because the Vent Hole isn't that big, there is plenty of gas pressure to force the patched ball/shot down the barrel.
Now is when we have to consider Newton's Laws of Motion. I'm going to list them, but won't go into great detail to explain them, as it isn't necessary.
- An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
- The acceleration of an object depends on the mass of the object and the amount of force applied.
- Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.
Law number 2 is very important to the question. The patched ball or shot column is the object the gas pressure has to move. It takes the most gas pressure force to begin to move it and less force to move and accelerate it down the barrel. Well, that's about as clear as mud, isn't it? OK, so let's give a somewhat similar example that almost everyone knows about.
Most of us have had to or at least seen someone or someone's pushing a car or other vehicle from the back. Those of us who have pushed the vehicle, know you have to push with the greatest force to begin to get the vehicle moving. Once it is moving, it takes less force to keep it moving and even going faster, once it has moved for a while. This is similar to what the gas pressure in a barrel does with a patched ball or shot column.
The gas pressure applies the greatest force on the ball/shot column near the breech where the "container" or space inside the barrel is at the smallest volume or least space inside the barrel. While the powder first begins to burn, it is building gas pressure until there is enough force to move the patched ball/shot column. Once the patched ball/shot column begins to move, that opens up the space inside the barrel behind the ball/shot column and the gas pressure begins to come down a bit once all the powder is burned.
So even though there is less gas pressure than what it took the ball/shot to initially move, the gas pressure continues to apply pressure against the projectile and the vent hole. There is less gas pressure as the "container" or open space behind the ball/shot increases in size as the ball/shot is driven, but there is still plenty of gas pressure to keep accelerating the ball/shot as long as the ball/shot is still in the barrel. Thus, the longer the barrel, the faster the ball/shot moves, until the ball/shot exits at the muzzle. (Now there is a length of a barrel that would be long enough to see the ball/shot actually eventually slow down inside the barrel, but that length of barrel is MUCH longer than any single person is going to carry.)
IOW, it is not the powder that continues to burn and causes the ball/shot to speed up inside the barrel, but rather the gas pressure from the burned powder that does it. So for our purposes, the longer the barrel we are willing to carry, the faster the ball/shot will travel inside the barrel and give the most velocity to the ball/projectile when it leaves the barrel muzzle.
Gus
P.S. Newton's 3rd Law is what gives us felt recoil after the shot goes off.
- 3. Whenever one object exerts a force on another object, the second object exerts an equal and opposite on the first.
Inside gun barrels, there is force applied to move the ball/shot and an equal amount acts against the gun and thus our hands or shoulder. However, since the ball/shot is much smaller in mass/weight than the gun, we are not propelled backward as fast and with as much force as the ball/shot is propelled out of the barrel.
Gus