Within the framework of a competition (i.e. ASF TRAP, SKEET), this official ISSF MARKER TAPE is mandatory. It aids the Referee in controlling the position of the gun. The tape must be permanently affixed to the shooting vest (i.e., outer garment) according to rules you may find on https://www.issf-sports.org .
 
Price : 5 Euros per unit. 
These Marker Tapes are also sold by lot of 5 or 10