This is a plug designed to go into the end of the muzzle on the DC-17 blaster. Some conventions require these, in bright orange, on any prop firearm, even if it's a sci-fi one like this. When inserted, it should fit snugly enough to stay in even when waving the weapon around, but not so snugly that it can't be easily removed for a photo and then re-inserted afterward.

For mine, I used bright orange ALS (for a blaster made from PLA), and it works like a charm.