Many horses get rubbed out shoulders, especially during Winter when we tend to have more rugs on them and remove them less frequently.
Interestingly, rubbed shoulders and wither sores are not usually caused by a rug that is too small, but rather one that is too large through the neck.
When the rug fits well it should do up fully closed and the front and sit well in front of the wither. If a rug is buckled too low then the pressure point is often level with the point of the shoulder and hence the constant friction of movement will rub out the shoulders. If the rug is allowed to sit back too far then it will create a pressure point on top of the wither which will move as the horses shoulders move - a bit like a loose shoe on your heel, and a sore will often result.
While shoulder gussets are a great asset in your top rugs or in summer sheets, when using them in under rugs the heavier outer rugs will cause the chest strap to ride up and the top of the neck to slip back often resulting in wither pressure sores. A good fitting neckline in your rugs should be all you need to stop rubbing.