There is a difference between the weapons you are familiar with and the weapons you are actually rated for.
Indeed - this is why it is a speciality skill. Once you have gained skill in one firearm, you can use them all with _some_ degree of familiarity, and have the option to specialise in more.
For everyone else, remember, these rules are _optional_. If you feature a lot of guns on your games, they may be useful. If not, you'll find the vast majority of weapons slide quite neatly into one of the original specialities.