If they're "true to life", then the horizontal component is constant and the vertical component is -9.8t. Unless of course you point upwards. Then the horizontal becomes initial velocity times the cosine of the angle between barrel and horizontal and the vertical becomes initial velocity times the sine of that angle minus 9.8t.
(Vi)sin(angle) - 9.8t
That all ignores wind. How they did it in game is probably a simplification as well for efficiency.