Someone asked for a Physicist?
O.K. Here I am. You will be sorry!
The answer is over in SS7MM ‘s reply in the Ramshot thread. BJ verbally got most of it right. As a side note it is interesting how much incorrect information is posted on the internet concerning work and energy in what passes for reference material.
First and foremost for all of the engineers - this is not a mechanical system. It is a chemical system just like a car. Energy comes from the combustion process. How much energy is in the powder and how much is released and converted into mechanical energy depends on barrel length powder burn rate, bullet weight, barrel diameter etc. Being an engineer also, I failed in my first attempt to analyze the problem because I assumed it was a mechanical system.
OK us lets us proceed to answer the question of free recoil versus shoulder recoil versus a brick wall.
Without resorting to any specific powder and bullet combination we will just use some typical numbers.
A primer is struck and ignites the powder which burns and expands at a rate sufficient to occupy the barrel and push a bullet out of the barrel at 3000 fps. In doing this the expanding gas applies pressure (force = mass times acceleration) in all directions. Some is applied to the bullet which moves down the axis of the barrel and a mechanical reaction occurs in the opposite direction if the gun is free to move (free recoil). The gas expands at a velocity that is controlled by the resistance of the bullet to moving down the barrel. This is the central point everyone is overlooking. The rate of powder burn and gas production in the barrel and the velocity of its expansion is the same no matter if the gun is attached to an F4 fighter jet going Mach 2 or is in a vise on a brick wall or in the extreme situation it could be mounted firing backward on a rocket sled which is a very funny situation of bullet falling on the ground. In physics we use different frames of reference called inertial frames to analyze this type of problem. This involves an observer involved with the situation and an independent observer standing off to one side.
The amount of time it takes to get the bullet out of the barrel by the expanding gases is the nearly the same (I said nearly) in all cases because the powder burns at the same rate no matter if it is moving or still. So the velocity of the bullet as it exits the barrel is 3000 fps if a person was sitting on the end of the barrel.
If a person is on the ground watching the gun being fired from a jet plane then the velocity of the bullet is 3000fps PLUS the speed of the plane. The pilot sees the bullet depart at 3000fps.
If a person watches a gun mounted backwards on a rocket sled and the sled is traveling at 3000 fps when the gun is fired the bullet will simply falls straight to the ground. The person on the rocket sled see the bullet depart at 3000 fps.
For all of you unbelievers, the next time you go deer hunting and are riding around in the back of your buddie’s pickup truck try throwing your beer bottle backwards and see if the bottle actually flies backward or if it actually flies in the same direction the truck is moving. Which occurs depends on how fast the truck is going and how drunk you are.
At this point in time those of you who are too bored to read anything else and are still with me, will probably understand that the movement of the gun during the time the bullet is in the barrel affects the velocity that a third party observer will see. So now lets get to free recoil versus shoulder vs brick wall.
Brick wall. Gun cannot move so bullet velocity is controlled solely by powder combustion process and exits at exactly 3000fps as seen by both observers.
Free recoil. Bullet in relation to barrel exits at 3000fps but barrel is moving to the rear at 4 fps (Bart B’s number). Independent observer sees bullet at 2996 fps. How fast the barrel moves to the rear can be calculated with any of the recoil calculators available on line. The one I have bookmarked is
Chuck hawks has these number for pistols
Shoulder recoil. The shoulder is simply addition of weight to the weight of the gun so there is more mass to move backwards. How much weight depends on how many Christmas cookies you have been eating. The shoulder slows the rearward motion of the gun down from the free recoil speed of 4 fps to let us say 1 fps (Monkey Blaster excepted – no shoulders big enough to slow it down). Bullet leaves the barrel at 3000 fps in relationship to the barrel but an independent observer standing to one side sees the bullet come out at 3000 – 1 = 2999fps.
Hope y'all have some accurate chronographs. [img]/ubbthreads/images/graemlins/grin.gif[/img]