The sniper shoulder is all for looks it does not effect you in any way. It does not give you extra bullets and neither does the shotgun bullets on the wrist. I hope this helps.
Yes
Yes, you are correct.
The only way to know is to pattern your shotgun with as many different kinds of CORRECT ammunition as you can afford. Firearms will often show a marked preference for a brand or weight of shot.
The second expression is much more common.
Yes, you are correct.
The butt of a rifle or shotgun- the part that goes against your shoulder.
Cheek
The correct way to wear a sash is diagonally across your body, from one shoulder to the opposite hip. This is typically done for formal events or ceremonies. Make sure the sash lays flat and is securely fastened to prevent slipping.
There are two joints in the shoulder - the 'main one' that joints the bone of the arm (humerus) to the shoulder blade (scapula) is called the glenohumeral joint. A smaller joint connects the shoulder blade with the collar bone (clavicle). This is called the acromioclavicular joint.
The spelling "shoulder" is correct (top of the arm, at the neck level).
In America I have always been tought to "pass like cars" or pass left shoulder to left shoulder.