When a firearms manufacturer makes a shotgun (or any other type of firearm, for that matter), the general rule of thumb is to make it so that it fits a large percentage of the customer population. In the case of shotguns, they assume that the buyer is male, right-handed, about 5 foot 9 inches tall, weighs between 160-170 lbs. or so, is right-eye dominant etc. (i.e.) the ergonomics of an average male human in the particular country that the firearm manufacturer is based in. So what happens if the buyer does not meet this standard -- for instance, the buyer could be left handed or above 6 feet tall or below 5 foot 5 inches and weigh more than the average weight, have a shorter neck etc. Female shooters are usually shorter, have longer necks, shorter arms, breasts, higher cheekbones etc. In such cases, the shotgun doesn't fit the customer so well.
We already studied some of the basics of stock design many months ago in this post. The reader is advised to read that first, to get familiar with some of the terminology of the various parts of a stock.
Some of the many factors that make a shotgun comfortable are balance, weight, cast-off (e.g. if a person has fat shoulders, the stock may be angled to one side to allow them to aim properly), length of the person's neck, eye dominance (right or left eye) etc.
In the 19th century, most firearm stocks were made of wood, so unless the stock was custom made to fit a certain individual, anyone who was not of average dimensions had to make do with a firearm that didn't handle so well. In the latter part of the 20th century, firearms manufacturers did make some improvements to help people who did not meet the average dimensions. One way they did this was by making stocks of different sizes to satisfy more of the population (such as guns made for women and youth models). Another way was by making adjustable stocks.
Click on images to enlarge
The first stock we see in the picture above is made by a German company called J.G. Anschutz, and the second one is made by an American company called McMillan. The stocks are made of different materials (laminated wood vs. fiberglass). However, they both feature adjustable combs and butt pads, that can be adjusted for different shooter preferences. Many shotguns also feature adjustable triggers that can be moved forward or backward.
In extreme cases, one may replace the adjustable stock with a precision fit stock, such as the one shown below:
Click on image to enlarge
With a stock like this, the user can adjust the comb angle (not just the height) to adjust for any face, adjust the recoil pad angle and distance to fit any shoulder, adjust for any cast-on or cast-off etc.
Another way is to visit a good gun-smith for a custom stock fitting. Any good stock maker usually has what is known as a "try gun" in the trade.
Click on image to enlarge. Image taken from W.W. Greener's The Gun and its Development 9th edition, which is now in the public domain.
This is a gun-maker's tool, that allows the maker to adjust the stock to any length, bend, cast-off and butt shape. This can be used to fit anyone who needs a custom built stock. The try-gun is adjusted until it fits the customer properly and then the measurements and positions of the various movable parts are noted and given to the stock maker to carve a custom stock to these measurements. Famous high-end gun makers from the 19th century, such as Purdey, Greener, Westley Richards etc. used such tools to make custom stocks for rich customers and still do so to this day.