Take the .45 SMG, it has "50+1(5)" listed under its shots. I know what the "50" and "(5)" mean, but what is the deal with the "+1"?
It means you can have an additional round in the gun's chamber, in addition to a full magazine. If this is explained in the Basic Set I can't find it, but it is on p. 79 of GURPS High-Tech:
Shots: Weapons that can load an additional round directly into the chamber (p. 88) have a “+1” appended to Shots.
This is noted as an addition to the statistics in the Basic Set. The terminology seems to be used to some extent in the real-world firearms community, since I've encountered it in a Forgotten Weapons video on YouTube.