Alright so this game isn't HELLMET. its actually Project Apex developed by ARCSOFT (the game still in development). The weapon and arms rig is entirely on the R6 character itself and camera is offset from the character's head. The recoil you are seeing is entirely animated.
To rig the weapon to the character. You can choose either left/right arm depending on the weapon or the torso to animate both arm movement easier.
For example (Video from ARCSOFT):
https://cdn.discordapp.com/attachments/1194244068352471080/1361125943669821670/joe.mp4?ex=68a7b051&is=68a65ed1&hm=4d72ccf7c484d33692c48d02226b9809fa87d2e5186e8d394ba5b071863fb317&