Im guessing shooter is what I would call a miniature? ie small bottle. As opposed to shooter type of cocktail you knock back in one gulp.

Depends on the alcohol. Gin I would probably mix with tonic. Whisky I may take over ice or in an old fashioned. The idea is to try it out before paying for a whole bottle. Like a sample.

In the blue collar area where I grew up it was always called a “shot”, and it was popular to stop in to the local bar for a shot and a beer. The shot was usually a bourbon or blend, served in a 1.5 oz shot glass; the beer was a 12 oz bottle, can or draft.

As far as how to imbibe the shooter in question: 50 ml is roughly 1.7 oz, making it slightly more than a good shot. Whether to mix or shoot would definitely depend on your intention and preference, and the type of liquor in the container.