243
Magnums usually shoot faster. Hence the term Magnum
A magnum gun is a gun that will shoot a magnum cartridge, and that cartridge will have magnum in its' name. It is just a moniker that infers more powerful cartridge, but the name itself has little or no significance...other than as a marketing tool. One that will propel a projectile at significantly more velocity than a regular one.
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.
With a tight bore barrel and sealed nozzle and higher spring
Answer depends on which cartridge is being fired, and from what gun. In GENERAL, the 22LR is ABOUT 1400-1500 feet per second. The .243, ABOUT 2900 FPS.
I would recommend a 223 or a 243 rifle or a 28 Gage shotgun.
IF a rifle is chambered in caliber .35y magnum, yes. Among others, Marlin made a lever action .357 magnum rifle.
Yes. Your revolver will fire either one.
The .357 Magnum was the first gun to use the term magnum.
Not safely. Despite the tales you have heard about this, the pressures of a .44 Magnum are well above the design pressure of a .410. WHEN (not if) the gun fails, someone is going to be hurt. No. Don't.
The short answer is no. You should not (and normally can't) fire any ammo in a gun other than what it is specifically designed to shoot. There are some exceptions, such as, you can fire 38 special in a gun designed for .357 magnum.