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Stock Fit. Part III. Cast-on, Cast-off.
Cast is the horizontal offset of a stock that allows your eye to line up with the rib. Viewed from above, barrels forward, stock bent to the right we call “cast-off”, and to the left “cast-on”. Cast-off or right hand cast is normally for right handed shots whilst cast-on or left cast is for lefties. This does not always follow for reasons we will come to later. Cast is measured as the amount the centre line of the stock is offset from the centre line of the rib and is measured at typically three points: comb, heel and toe. A fourth point is sometimes given as cast at face but as the exact position of the face is indeterminate this is more theoretical than practical.
Although cast is the offset of the comb at the centre line it is important to note that not all stocks are equal. Comb profiles differ from gun to gun and whilst some will have a sharp narrow comb, others may be wider and flatter on top. The cast, as measured, may be the same on two guns but due to differences in comb width the effect on the user will be very different. If the comb profile is wide, especially towards the waist of the stock, it will keep the face from coming into the gun with the result that the shooter’s eye will be over to the left of the rib (right hand shooter). In some instances the shooter will roll over the top of the comb in an attempt to line things up visually.
This apparent lack of cast in a wide stock is sometimes compensated for by increasing the cast, usually achieved by bending the stock. The problem is that bending the stock to give only 3mm more cast at the point of contact with the face will result in the cast at the heel of the stock increasing by possibly double that. This may result in the heel of the stock being positioned more out on the upper arm instead of comfortably in the natural shoulder pocket of the anterior shoulder muscles. The upper arm is not suited to absorb recoil and this can lead to contusion injury, a nasty bruise in simple language. The position of the natural pocket is easily identified by rolling the shoulder forward with the fist of the other arm pushing on the front of the shoulder just in from the top of the arm. As the shoulder rotates forward you will feel the pocket open and the shoulder muscles push the fist away from contact with the bone. The sole or butt of the stock should drop into this pocket with the heel well up towards the top of the shoulder but not above it. If you have too much cast at heel you will find it difficult to achieve unless you are standing very square to the target.
Individual stance governs the amount of cast needed. If you stand very square to the target you will need more cast than if you stand with your left shoulder towards the target (right hand shooter again) with the gun mounted across the chest. Those shooters who adopt this so-called “rifleman’s stance”, may find they need left hand cast in order to prevent the stock mounting out on the upper arm, even though the gun is mounted to the right shoulder (the reverse being true of left hand shooters). This is much more common than you may think as hinted at in the first paragraph.
Cast at toe is to allow the stock to follow the natural angle of the shoulder pocket and prevent the toe of the stock digging into the upper chest. This dimension, together with pitch, is important for comfort, especially for large chested individuals. Be careful not to go to extremes with cast at toe or the pitch angle because the bottom half of the sole must have support to provide a stable platform. Secondly, to effectively absorb recoil without local contusion damage, it is essential that the load is distributed evenly over the whole surface of the sole. Contact with only the top half of the sole will increase recoil in that area. The toe needs to be only cast outwards slightly to prevent it digging into the upper chest whilst still maintaining contact and providing support. Typically, cast at toe need only be an average of 4mm more than cast at heel although this will vary with the build and stance of the individual.
It can be seen that although the primary purpose of cast is to put the eye centrally behind the rib there are other important aspects to be considered. A stable consistent gun mount which minimises recoil damage and discomfort will have an equal effect on individual performance.
The determination of correct cast requires much more than just checking where the eye is in relation to the rib, although that is a good starting point. Individual style, build and stance of the shooter and how the load is carried through into the shoulder are extremely important. That and interpreting cast dimension from one gun to another, with perhaps a different comb profile, requires knowledge and experience.
Stock Fit. Part IV will look at comb height. If you want to read Part IV as soon as it is published just subscribe below to get an automatic email notification the moment it goes live. If you change your mind and don’t want any more notifications, no problem, every email carries an ‘unsubscribe’ link. Click on that and you will never here from us again.
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