Fps issues (i think)

Ok so when i use certain guns, like ak or mgs, when i shoot the bullets dont stay in the crosshairs, so it makes it hard to aim, also my fps is like 25-55 but i dont think that is right cause the game runs fine at any graphics setting, sorry if this is hard to understang, but does any1 have that crosshair problem?

You need a better P.c and better FPS