What is the scientific aspects of gun(s).

Well, if you are talking about guns as in firearms, and how they work, it is actually pretty interesting. 

Lets take a Glock-18 for example. It's hammer is internal. With a more common pistol, the m1911 for example, the hammer is external. A bullet is made up of three parts. The shell (cartrage), the gunpowder (known as black powder), and the bullet (the projectile, usually lead). The gunpowder goes inside the shell, then the bullet is attached to the top. When the hammer of a gun hits a pin, which hits the back of the bullet, the spark ignites the gunpowder and the combustion caused the bullet to fly out of the gun. The shell is then disposed of (usually out of the side or top of the gun). The same principal works with ar-15's or m4-a1's. The hammer is internal in those guns, and the hammer automatically goes back from the kick of the explosion (inside the bullet its self). There is a piston along the barrel, and when the bullet flies through the barrel, the air pressure goes in the piston, and it is enough force to cause the slide to go fully back, and then eject the shell at the same time it loads another round. 

I hope this was helpful to you.