How do we get accuracy if the gun starts to move before the bullet has left the barrell ?

Practice, practice and more practice. Seriously, its a short period of time. Thats the reason for good, solide bedding. Or at least one of them.

From a physics standpoint, there are two parts to gun recoil. There is the part where a 180 grain bullet is heading down a 24" barrel at 3000 fps, and driving an 8.7 lb. rifle to the rear ata calculated counter-reaction. Regular physics stuff. And then there is part B: when the bullet breaks its seal at the muzzle and creates a jet engine effect from the gasses exiting the barrel behind the bullet. Part B might be close to half of what your shoulder feels, and is the part of recoil that a muzzle brake serves to mitigate. It is also why the WSM calibers seem to kick less - they use less powder to develop the speed in their short, fat case; and the pressure curve of the expanding gasses is steeper - causing a lower pressure "jet engine effect" when the bullet leaves the muzzle.

Everything i can find about recoil refers to it as a function of momentum. That would imply that recoil is developed as momentum is developed. So IMO recoil must start to build at the same time the bullet starts to move. If a 100gr bullet goes from 0 to 3000 fps in the time it travels down the barrel then recoil would be developed over the same time. I don't think recoil could happen after the bullet leaves the barrel because the bullet dosen't accelerate after it leaves the barrel. I think the inertia of the gun at rest resists the recoil for the .001 or so seconds that the bullet travels down the barrel.

so if I were to use one of those fancy gun rests for sighting in rifles, the one where it straps the gun down like a vice, would it then shoot different if I now hold it on sandbags, or just rest it from a blind or off a bipod.

My browning shoots different from a vise than from a sandbag. Probably be more noticeable in lighter weight guns.

Because even gasses have mass. When you apply a force to accellerate any mass, there in an equal force applied in the opposite direction. Think jet engine.

As for when recoil begins, it begins the instant the mass begins to move. The effect of the gaseous mass accellerating down the bore is also felt immediately because the gas accellerates with the bullet.Also remember that while thegasas a whole may not be moving very fast while the bullet still contains it in the bore, the individual gas molecules are in a veryexcited state becauseof the high temperature and are moving veryquickly.Pressurequite literally is theaverage kinetic energy transfered bythe impact of moving molecules with the surface of an objector containerthey interact with.

Mike

Deleted

For what it is worth, there are some interesting photos at www.african-hunter.com/recoil_vs_accuracy.htm. (You can get there through their archives if I botched the address.)

Stock build, the width of the stock and recoil pad and of course rifle weight have a whole lot to do with recoil for me. My son's 308 Tikka kicks more than the 338 Win Mag I had. I believe there is recoil as soon as the bullet leaves the casing, and the barrel reacts as the bullet goes down the bore. But the rock & roll really starts, as far as I know, once the bullet leaves the barrel and gasses escape freely. Otherwise, we would not hit much. But I am open to other thoughts. What the heck. It is an important topic.