The Star Trek Tracer Gun was one of the most-used toys at my house in the mid-to-late 1970s.
If memory serves, at one point I had two and would have frequent battles with my sister and friends. The surviving gun is this gold model. I believe the other was blue. As you can see this one sustained some battle damage which required a bit of Jeffries' space adhesive (a.k.a. masking tape) to repair.
The discs, though colourful, were fairly small and would often end up behind couches and chairs or, if playing outdoors, lost forever. Hence the need for disc refill packs, such as this one.
Here are a few of my remaining discs.