You’re using the wrong scene scale, in my scene the rifle was 1.5cm long. To fix it you set the scene unit scale to metric 0.01, then scale the rig up 100 times. After that do Apply Object Transform (with spacebar), ticking Scale in the lower left. Your animations will get messed up, to fix it, select the rig and open the graph editor, there click the magnifying glass and type “location”. Then to the right click the pivot point and change it to 2D cursor. Then select all keyframes and scale by 100, pressing Y to constrain the scale vertically.
Your rifle rig is also offset in the scene which could cause problems, you can fix that by pressing Ctrl+C to set the 3d cursor in center, then selecting the armature and doing Snap Selection to Cursor, Apply Object Transforms ticking Location Rotation, then do the same for the mesh. You may need to reset the location keyframe for the body and insert new keyframes.