The keyboard can be mightier than the shoulder fired gun.
Remarkably, we don't actually rest the stock butt on the shoulder, it's on the pectoral muscle which is responsible for some of the complex movements possible with the shoulder joint. This is usually considered a chest muscle.
The interface between the gun and the pectoral muscle needs to be fairly tight within reason, and I find tensing that muscle up while pulling the gun in provides this.
Do we move the shoulder forward thus 'stepping into' the shot? Possibly, but it's also required to flex the elbow inward thus pulling the gun into the pectoral muscle.
The goal is repeatability. The mount should be essentially identical with any gun in the rack and any shirt or jacket.
Your starting point may well vary depending on the sport. While it's safe to assume a 'ready' position behind a trap with the gun horizontal, generally your muzzle will or should be quite a bit higher behind a pointing dog. We lose fewer dogs and hunters this way, and it should be just as easy to get a good mount from a sky pointing position as from any other.
