The sole purpose of a mid bead is to check the gun mount. After that, it is of no value.
One bead behing the other versis a figure-8 involves an adjustment of the rear sight (the eye) on a shotgun. A figure-8 bead alignment raises the point of impact somewhere around 4" compared to inline positioning.
Parallel comb stocks are becomming more common for other-than-trap shooting disciplines. As was mentionned, they can make gun mounting more consistent and accurate as well as avoiding the higher part of the stock (comb) comming back during recoil and mashing the cheek.
The one difficulty in their use is that unless the drop at the crop dimension is correct, there is no way to compinsate, unlike a gun with a rising comb where the cheek can be positioned farther forward or back to adjust the drop at the comb dimension (to get the eye vertically aligned with the rib.)
The above concern is offset by the advantage involving the head's being able to be positioned anywhere along a parallel comb with no change in vertical bead alignment. With a rising comb, moving the cheek forward or backward to bet the eye aligned with the rib can screw up the effective length of the stock ( the length of pull dimension.) There is always the question, will the cheek stay in that position during swings to targets. If the cheek moves back it makes raising the head to see the target more likely.
When it is unnaturally forward, there is the risk that the cheek will slide back during swings. That will cause the bead/rib to be blocked by the receiver/action and also invite head raising to see the target.
Shoot Well,
Rollin
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