When does a gun recoil ? Is it when the bullet leaves the barrell ? what causes recoil,
size of bullet or powder burn or what ?
can some explain all this to me.

Well, its the pool ball effect in physics. Two matters pushing equally against each other. The powder burning rate can cause a faster impulse of recoil or slower with the same overall energy. So two guns shooting different powders but getting the same recoil, may feel different with the same ft-lbs of recoil.

What I nor my old professor could figure out is why the recoil formula put powder wieght as so much a dominant factor. Yes, its part of the load leaving but also burning up. Bullet wieght makes perfect sense.

In a way its simple but in a way it isn't. Its easy for me to understan the equal and opposite reaction part and the part bullet weight plays on it. On the other hand FELT recoil is another question. Why do some guns who 's recoil on paper is less,hurt the shoulder more than those with less on paper recoil? I am sure its weight of the gun, shape of the stock, the kind of recoil pad and so fourth. Strange to me is the fact tha I have shot many many rifles, some bigger by far than the 7MM Rem Mag. However the 7MM Mags seam to hit my shoulder harder than the 300 and 338 Mags. Is it recoil speed? It part of it in my head? Whatever it is it hurts.[:@]My 416 Rem mag is the only gun I have owned that hit me much harder than the 7 Mag.

I know the heavier bullets create more recoil, but I seem to feel it more when I use faster burning powders. Maybe slower powders accelerate the bullet over longer time and spread out the recoil over longer time?

I know powder charge, bullet weight, bullet diameter, and gun weight all play a role. If you want to know how much recoil your gun has www.shortmags.org has a recoil calculator, just put in your info and it will tell you. My 300WSM has 30 ft-lbs and my 30-06 has 21ft-lbs.

A lot of people think that pressure affects recoil... not true pressure occurs in all directions, so it has no effect on recoil. One thing to keep in mind is velocity... the faster the projectile, the sharper the recoil. This is why shotgun recoil is so much different from rifle recoil.

bigcountry - I agree that the weight of the propellant (since it is largely in gas form) does not play nearly as large of role in recoil as many calculators indicate

Bingo, you got it. thats the reason sluggun has such a rocket effect. Usually uses much faster powders. Where my 300RUM didn't but more of a slow hard roll. The actual acceleration has to have an effect on felt recoil when graphed over time.

But when does the kick occur ? is it when the bullet leaves the barrell ? is that when the explosion happens ? it seems thats the only way you would have accuracy, when the bullet leaves the barrell.
I mean it couldnt kick when the bullet leave the shell, or could it ?

In principle when the projectile leaves the case is when the kick starts. you have two separate objects separating

Bullet leaving the case would be my guess as well. All guns have a little free bore dont they? I love my 7mag accuracy wise.....but recoil wise it WILL stay in the cabinet. That thing got a sharp punch that I just dont care for. Its a pretty Weatherby or it would go down the road....lol.