Jim,
I think that you are not defining recoil in the same way that I am and in addition you are failing to standardize your argument. This argument depends on the type of action you are using. Let me state these facts as they apply to recoil. Recoil is any movement of the weapon that can be felt by the operator.
1) In a gun that has a locked breech that is not recoil/blowback or gas operated, there is no recoil if you if you block the barrel and pull the trigger if certain conditions are met. As long as the lugs and barrel are stong enough to hold that pressure, nothing happens, you are correct in this instance. If the lugs shear or the barrel bursts, you will enjoy recoil as the gas quickly forces its way out. The faster the gas leaves, the more you will feel it.
2) In a gun that has a locked breech that is gas operated, the gun will have a recoil and cycle the action if the barrel is blocked and sufficient gas is introduced into the system to unlock the breech. This how a blank gun works; you plug it, shoot it, and watch it cycle. The felt recoil will be due to the cycling of the action which is going to either short cycle or over cycle the action. There is 1 single point that could result in a flawless cycling that would be transparent.
3) In a blowback/recoil operated gun, if the barrel is blocked and the gun is fired, there will be recoil if sufficient gas is produced to unlock the breech from the barrel.
Let me disect your argument...
If you block the barrel so the bullet cannot move, there is NO recoil.
That is only true for case 1. You can remove the bullet if you want, you don't need it in any of these cases. Crimp the end of the catridge and call it a blank.
In case 2 and 3...
Gas pressure is not a factor, since it operates equally in all directions in the chamber;the net effect is zero. There is no recoil, no movement of the slide or barrel, and the breech remains locked.
In cases 2 and 3, this is incorrect. The pressure is indeed acting in all directions in the chamber, except that it isn't confined to the chamber. If you have a bullet in there, it will fly to the end of the barrel and stick there (bulging the barrel where it sticks). The gas will then be operating on all surfaces equally. However in case
2) The gas will seek an area of low pressure (take a M16) and go into the gas tube. It will then operate on the bolt carrier with the same force it is applying to the barrel, the barrel obstruction, the chamber, etc... If the pressure is big enough, it will operate that action. If not, your gun will get hot and eventually cool off. This is exactly how that gun works in the first place. You simply need a bullet to keep the pressure in the vessel (right after it passes the gas tube hole) as the gas quickly fills the tube and operates the action.
3) The gas will, once again, create equal pressure on all the surfaces it can. In the case of the breech face, it will push back against it via the case head and if the pressure is enough, it will unlock the breech and cycle the action. Just like if you grab the slide and pull it back, that gas can force it open. The breech is a degree of freedom in the system, just like that operating rod is.
If cases 2 and 3 were not true, then pneumatic cylinders wouldn't work. You squeeze fluid into a cylinder under pressure, and if something can move, it does. As a matter of fact, it is a *really* efficient way of moving something. We can easily take our guns and turn them into pressure vessels. So here they are
1) A gas cylinder with an inlet but no outlet
2) A gas cylinder with an inlet and an outlet piping with valve that is operated by a rod, with a piston facing the gas, held in place by a spring.
3) A gas cylinder with a simple pop off valve held down by a spring.
Pump in extra gas and #1 explodes. #2 will eventually overcome the the spring and open the valve. #3 will pop once the pressure overcomes the spring. This is exatly how huge gas compressors work. Their pistons have springs inside of them that give when a certain pressure has been reached so that gas can vent by them.
and a cartridge with no bullet is fired, the gas generated by the burning powder escapes through the opening. Since that gas has mass, there will be recoil in the opposite direction.
That is exactly the principle I have been trying to get across. I don't know why you recognize it there but don't recognize it in cases 2 and 3. The fact of the matter is that guns have been created that don't require a bullet moving to cycle the action via blowback/recoil or direct gas. There is an instance where a 50 cal machine gun was cycled with a 22 bullet via gas pressure in the chamber. No way in hell the quantity of gas from the 22 could operate it in the normal gas tube fashion, but by using pressure in a very wise way it was done. The original Garand operated on the recoil of the primer! It moved a couple hundreths of an inch as the pressure of the burning powder forced it back in the cup.
You can prove this is true if you have a threaded barrel. Put the gun in a vise, thread a pressure fitting on there, crank it up to 35,000 PSI, open the valve and watch your 1911 action operate.
* edited: It's a gun, not a bun!