A ray gun may refer to multiple things in the series:
Paw guns[]
is built into both of the hands (or front paws) of Kat.
At the start of You Scream, I Scream the parenthesized closed caption "(ray gun shots)" is used to describe the sound it makes.
Kat stabilizes his left hand by holding his left wrist with his right hand and closes his left eye when aiming with it.
He then switches to firing with both hands, keeping both eyes open.
The gun fires red rays manifested by the distal half of his index finger glowing red with a point of white light at the tip of it.
The rays can appear briefly or remain for some seconds as a cutting ray he can swipe across to strafe a area.
In Kat of Diamonds he uses it to repair his blackout device and wears a welding mask while doing so.
Shrink ray gun[]
In The Incredible Shrinking Coop Coop initially calls it "the weapon" when telling Kat to drop it, and then "that ray gun" when asking where it went.
After discovering what it does Coop later refers to it as "that shrink ray".