What is scope height?
Most ballistics applications will ask for this measurement to give an accurate prediction of bullet trajectory. Scope height refers to the measurement between the centre of the rifles bore to the centre of the scope atop it, often referred to as “line of sight to bore”.
Hunting bullet trajectory
To understand the effect of scope height, it is first important to understand the trajectory of a bullet in relation to the “line of sight”. The first rule of physics concerning light is that it travels only in a straight line (though it can be refracted easily, hence heat haze etc). So when a shooter looks through a scope he or she is looking down a perfectly straight line. The bullet however does not travel in a straight line; ignoring the Coriolis effect and wind the bullet will be seen to be dropping instantly after leaving the barrel due to the influence of gravity – so in effect the barrel is actually pointing upwards very slightly in order for the bullet to meet the scopes line of sight.
In simple terms below are six stages of a typical centrefire hunting bullets trajectory to consider – for demonstration purposes the scope height will be 2 inches (centre of barrel to centre of scope), on a .243 Winchester Rifle using 100g Soft Point Ammunition:
0 yards – The bullet leaves the barrel and at 0 yards will be 2” below zero
50 yards – The bullet is travelling up to where it crosses the line of sight the first time – this point is known as “Secondary Zero”, and typically is somewhere between 40yds and 65yds for an inch-high-at-100yd-zero on most centrefire rifles
75-100 yards – The bullet has continued to raise above the line of sight by approximately 1”
150 yards – The bullet has begun to drop back down towards the line of sight but is still approximately 0.8” high
175 yards – The bullet is back down to its Primary Zero (where the bullet crosses the line of sight for the second time)
225 yards – The bullet is now approximately 2” low of Primary Zero
Illustrated in chart 2 is the fact that the height of the scope correlates with the trajectory the bullet must take, and that this in turn alters the Primary and Secondary Zero.
Maximising “Point Blank Range” – High or low mounts?
To know the Point Blank Range of his or her rifle, a hunter must establish the “kill zone” or “kill window” for the intended quarry – with deer and larger animals this is usually 4” for a chest shot (to strike a vital organ), with rabbits and foxes this is usually 2” for a head shot.
“Point blank range” is the range at which you can put the cross hair on the centre of the target and the rise and fall of the bullet does not exceed the size of the kill window – i.e. for a 4” kill window the bullet is no more than 2” high or 2” low of centre. This means the hunter can simply “point and shoot” at the quarry without having to account for the drop of the bullet.
As the charts show, on a centrefire rifle high-mounts can be beneficial when shooting animals with large kill windows (4” or more), as the PBR is extended – whilst the trajectory is more pronounced the bullet remains in the kill window for a longer period. This has the additional benefit of allowing for large objective scopes to be used, which aid light transmission on higher magnification settings.
If however the quarry or kill zone is smaller, it becomes very important to keep the line of sight to bore as small as possible by using low-mounts. This is why 32mm and 40mm objective scopes are so popular for rimfire and “long range varminting” applications elsewhere in the world; they allow the scope to be mounted very close to the barrel without the objective lens being in the way.
As a general rule a hunter should use mounts that result in a line-of-sight-to-bore (or scope height) that is no larger than the kill-window of the intended quarry.
As well as considering the effects of scope height, it is important that a shooters cheek can make consistent and comfortable contact (or “weld”) with his or her rifle stock – whilst many manufacturers now include adjustable cheek pieces with their rifles, the majority do not.
Most rifle stocks are not suited to high mounts, as the shooter will be left balancing the bottom of their chin on the top of their stock rather than resting their cheek lightly on it. This can be easily remedied with a neoprene or leather comb-raiser, however if a large objective scope & high mounts are not necessary then it is preferable to simply use a mount & scope combination that is suitable for the rifle.