He couldn't let go of the gun. You might want to learn a little about human anatomy. All the muscle and nerve groups in your limbs inter-work with each other and if you have major damage to a limb close to your torso, everything past that injury will no longer function properly if it is able to function at all.
He's only missing part of his arm. Also give a NSFW warning on that image.
He didn't have a choice at that point. His tendon was shot to hell and hindered his ability to release the weapon.
He couldn't let go of the gun. You might want to learn a little about human anatomy. All the muscle and nerve groups in your limbs inter-work with each other and if you have major damage to a limb close to your torso, everything past that injury will no longer function properly if it is able to function at all.