Yes you can you can shoot 3 or 2 and a half in
Only it if is marked so.
In general, no. Unless the gun is marked for 3 inch (or even 3 1/2 inch) shells, you should not attempt to fire 3 inch Magnum shells, even if they seem to fit. However, if you have a gun made for 2 3/4 inch shells, and have 2 3/4 inch "magnum" shells, IF your gun is safe to shoot, it should be safe to shoot those "baby" magnums.
No. .22 Magnum may only be fired in a gun Marked .22 Magnum or .22 WMRF. Different length and diameter- as well as power.
The 12 g Marlin 200 single shot shotgun SHOULD be marked on the barrel, something like "model 200 12 g 3" Modified" That means it will fire the 3 inch magnum shotshell, and has a modified choke. If you have a shotgun marked 2 3/4, do NOT fire a 3 inch magnum, even if it will chamber. The forcing cone will not have enough length for the shell to open, and it will cause unsafe high pressures. Every 200 I have seen (a limited number) WAS chambered for 3 inch Magnum shells.
A 41 magnum caliber revolver will shoot 410 gauge shotgun shells but is not recommended. The shot will lead the barrel rifling. There are also adaptors you can put in a shotgun that allow you to fire pistol/rifle ammunition.
Yes
http://www.wikihow.com/Shoot-a-Shotgun
If the gun is marked 2-3/4 on the barrel, that is the maximum length shell that the gun will shoot.
NO
Short answer, NO. It should be marked as .22 s-l-lr (short, long, and long rifle). It will not be marked .22 WRM (Winchester rinfire magnum).
If your Iver Johnson is marked for the .22 WMR.then yes,if it is not then do not attempt to load or shoot .22magnum ammo in your rifle.
Your question needs some work- as written, has no meaning. A shotgun marked 3 inch can shoot 2 3/4 inch shells, or 3 inch. A gun marked 2 3/4 inch can only shoot those- no 3 inch shells.