DPI - I believe setting your DPI higher enables you to make more precise movements.
EDIT: I am currently using a Dell mouse that came with a computer my brother purchased a few years ago. Fun times! XD
I got the shotgun in 3 or 4 days I believe. My AK-47 took me forever because there is alot of assault rifles. I also have the Dragunov, I got the red tiger on the r700 and the M40A3 no problem, but the Barrett and the other two were hard for me.
In short, if you want a golden gun, start with the shotty, play 24/7 Shipment servers. ^.^