Well here are some of the basic things that will be different.
Effects such as lightning, fire, poison and stuff like that.
Attachments for the guns that are basically anything you can think of.
The colours/skins for the gun.
The actual shape of it.
Fire rate, accuracy and stuff like that.
EVERYTHING is different, like how many bullets you shoot before you reload, which will change the appearance of the gun's cartridges or how it reloads ect.
With all of those changes come a different looking gun, such as if the accuracy would be increased for a shotgun, the barrel would be smaller (Not sure if its smaller or bigger for better accuracy :C)
But yeah, there are TONS of new guns you will find, most of them having drastic changes then others you have saw.
The company created an AI and fed it a whole bunch of stuff, and then it creates millions and millions of guns.