If you have problem getting good looking gun use higher Control weight: 1.2-1.5 (only for Lineart)
If you have only one ControlNet unit go to the Settings > ControlNet and set "Multi-ControlNet: ControlNet unit number" to 2 or more. Click "Apply settings" and restart UI.
In ControlNet unit 0 use Preprocessor: none and Model: openpose. Drop openpose image here and enable it.
ControlNet mode for OpenPose set to "Balanced".
In ControlNet unit 1 use Preprocessor: none and Model: lineart. Drop lineart image here and enable it.
ControlNet mode for Lineart set to "ControlNet is more important".
If you want to use img2img (which I recommend) use img2img + ControNet Lineart with Ending Control Step set to 0.8.