Here's a thought experiment.
Imagine the bullet, just as it is about to exit ("uncork") the muzzle, as a valve. What will happen when the valve is opened, and the pressure inside the barrel is released (i.e., the mass of the propellant gases is allowed to accelerate to its maximum velocity)? Will additional recoil be produced, or not? (You might want to consider whether there really is residual pressure when the bullet is at the muzzle -- I think you'll discover that there must be considerable pressure remaining inside the barrel.)
Here's another way to investigate the "rocket effect" (not "jet effect," because the cartridge contains propellant -- fuel+oxidizer, not just fuel like a jet engine).
Go to any of the recoil calculators which can be found on the web. Here's one of them that allows you to compare two loads at once:
http://www.handloads.com/calc/recoil.asp
Now, if all of the recoil in a firearm can be accounted for by the combined mass of propellant and bullet (at the same muzzle velocity and rifle weight), then the calculated recoil should be the same for, say, a 100gr bullet with 30gr of powder and a 95gr bullet with 35gr of powder, since they have the same momentum (mass*velocity). This is what the "no rocket" crowd is contending. (I used 3000fps for the muzzle velocity and 10 pounds for the rifle weight, but the exact numbers don't matter as long as they're the same for both loads.)
When you run the calculator you will see that an added grain of powder produces more recoil than an added grain of bullet weight. The law of conservation of momentum tells you why this is -- it's because the velocity of a grain of powder (gases) is greater than the velocity of the bullet. Of course, this can't be true until the bullet leaves the barrel, allowing the propellant gases to accelerate to their maximum velocity.
The "rocket" phenomenon produced by the accelerating gases as the bullet leaves the muzzle is known as secondary recoil. You don't feel it as a separate event because it only happens a millisecond or so after the primary recoil produced by the momentum of the bullet and propellant before the bullet leaves the muzzle.
This is also why muzzle brakes are more effective (in terms of percent recoil reduction) for cartridges with lots of propellant mass relative to bullet mass (which doesn't describe a .416 Rigby!). The brake can't do anything about the recoil produced by the bullet, because the bullet has to go downrange. But the brake can turn the momentum vector of the propellant gases in some direction other than downrange, and a grain of gases has more momentum than a grain of bullet (once the bullet gets out of the way).
So understanding recoil really is rocket science ...
Toby Bradshaw
baywingdb@comcast.net
