Franchi;
The bore of a 12ga shotgun is .729". That amounts to an area of .417 SqIn (.729² x Pi/4). If a load peaks at 10,000 psi then for a "Breif" instant there is a rearward push on the gun breech of 4,170 lbs ( more than 2 tons). This is incidently what causes "Recoil Operated" firearms to operate.
"IF" when the charge left the bbl it resulted in pulling a complete vacuum in the bore for the air to "Rush" back into (Not at all likely as the gases are still exiting after the projectile has left) then consider that even at a high barometric pressure of 31.25 inches of mercury atmospheric pressure is about 15.34psi. Again in tha 12ga bbl of .417 SqIn this would result in a force when it hit the breech of about 6.4lbs. The gas thus has over 650 times the force of air pressure.
As to auto pistols remember the slide moves indepently until it comes to a halt at the rear. It thus slides along it's axis as the upward pivot from the grip is not there yet. I pulled the slide from a 9mm auto & it weighed 10.5ozs or 4,593 grains. Figuring a 115gr bullet plus half of a 6gr powder charge (the powder gas fills the bbl so it moves about half way until bullet exit) gives 118 grains. From the base of the bullet to muzzle is about 5". 118/4,593 x 5 = .1285". The slide will have moved just over 1/8" (.125") whn the bullet exits the bbl, but in that movement the momentum which accounts for the recoil has been imparted to the gun.
As to hatcher I have no idea how many lies he may, or may not, have told but this is not among them. All of this is well proven, I quoted him more for "The Quote", which I thought was so well put. Another of his quotes I love was when he said "That sounds good, When you "Say it real fast", but it won't stand up to inspection".
The statement of the recoil being caused by the air rushing back into the bbl is in that exact category. Matter of fact it is "Hotter Air" than even the hot burning gases exiting the bore.
Picking ones self up by their boot laces is of course in no way similar to igniting a charge of powder between "Two" objects (Sounds good though "IF" you say it fast). The powder charge puts an equal force in both directions & "On Both Objects". In W Greeners book "The Gun" 1834 he mentioned putting plugs in from both ends of a bbl with powder charge & touch hole in the middle. Guess what, it blew them both out opposite ends. The reason the charge moves faster & farther than the gun is simply because the gun weighs so much more.
