Clifford almost got the answer. Assuming we mean "launched from a Winchester" and not eaten by a raincoat, the trick is to suspend the rifle with the barrel horizontal, from two wires of equal length (the longer the better at least 2 or 3 meters) and add a substantial mass, say 20 kg of lead. A long photographic exposure, or a tape measure, will show you how high the rifle rises as the recoil turns into a swinging motion.

Then if the total mass of the rifle plus ballast = M and the maximum height of the swing is h, the recoil energy was M*g*h = 0.5 MV^{2} where V was the initial speed of the recoil.

Now if the mass of the bullet is m and its muzzle velocity was v we know that MV = mv.

and since the bullet accelerated from rest, v^{2} = 2as where s is the length of the barrel and a is the average acceleration

so a = (M*g*h/m)^{2}/2s

thus F = m x (M*g*h/m)^{2}/2s = (M*g*h)^{2}/2ms

with no sophisticated instrumentation required.

If you aren't happy about measuring the upswing of the ballasted rifle, you can get a rough estimate of F by ignoring air resistance and finding where the bullet hits the ground from a horizontal shot. I'll leave the arithmetic to you!

That method measures the momentum of the bullet and exhaust gas. Most ballistic pendulums do the same thing, except they measures how high a wood block pendulum swings after capturing the bullet. It's important to block the exhaust gas from the muzzle from pushing the wood block, and the pendulum must capture the bullet, not deflect it.

A more modern device to measure bullet velocity is the

ballistic chronograph. From the velocity and bullet mass, you can calculate momentum and energy.

The OP asked about force; that is not usually a concern. You can calculate the

**average** force if you know the velocity and the time in the barrel. However, there is a time lag from the time the firing pin hits the primer and the time when the bullet begins to accelerate, so the measure is not accurate. You can

**assume** constant acceleration/deceleration and calculate force based on the distance traveled during the acceleration or deceleration.

It is possible measure the instantaneous force pushing the bullet thru the barrel, using a special

laser ruler which measures the position of the bullet every few microseconds in the barrel. This can be done by reflecting the laser off of a sacrificial mirror so it looks down the barrel. This kind of measure is helpful for optimizing the powder load when reloading ammo.