Depends on the model. It depends on who made the BB gun.
Take the bb gun apart, memorize or take a pic of the inside, find the jam ,and put it back togethor
The Colt 25 airsoft BB gun cost between $10 - $15 depending on the model.
Not intended for user to take apart.
Colt makes a 1911-A1 Pellet gun made by Umarex. I know of no BB gun unless you are asking about an Air Soft gun. The Pellet gun is very good. I can not answer about an Air soft gun.
You don't magnetize the BB's. The Magnet is inside the BB gun. The gun has to be taken apart and the magnet that holds the BB's replaced. Not all designs use this method of holding BB in place.
About 100yd
Colt makes several different models. Which one are you asking about?
The Colt 1911 A1 is rated at 395 FPS (Feet Per Second)
The magazine is not meant to be taken apart and will likely shatter plastic if you try this
6mm
If this is an Airsoft gun then 6MM BB's should fit.