For convenience of description I will call this elevation of the line of the bore The Line of Elevation.
I shall also term that part of the circle between the horizontal and perpendicular above, the Upper Quadrant; and that part between the horizontal and perpendicular below, the Lower Quadrant, and use the figures on the dial of a clock to indicate the positions of hits on the target.
A gun is Canted when a perpendicular line drawn from the line of the sights will not intersect the line of elevation.
"Shooting Straight" (an expression legalized by use) means hitting a target at the point which is in line with the sights.
When a rifle is canted, no matter how little, it will not shoot straight.
Unless the contrary is stated it is always supposed that you are firing point blank at the range for which the rifle is sighted.
A bullet projected from a rifle canted to one side, say the left, at right angles to the upright position, will hit half-past seven o'clock, because the line of elevation throws it to the left just as much as gravity pulls it down, to counteract which influence the rifle must be aimed at half-past one o'clock.
If the rifle be held upside down it will shoot very low, because the line of elevation and gravity both act to depress it.
Held in any position between these, the two regular causes of deflection, gravity and elevation, will influence the shot; counteracting or aiding each other with mathematical precision, as they approach or depart from each other; in the upright position, just counteracting; in the inverted position aiding each other; and in the side position pulling down at an angle of 45 degrees.
In shooting at point blank range horizontally, the line of elevation is just counteracted by gravity.