If you ever have the misfortune of being within arms reach, or are able to reach a shooter-killer with your hands do this;
Grab the weapon with your weak hand and direct it muzzle upward and away from you if you can - and hit him/her as hard as you accurately can in the throat. Strong hand outside edge, first finger knuckles, the web of your hand between thumb and index finger, outside wrist, elbow, whatever you can or prefer. It will very likely disable him/her instantly.
If your positioning, or natural reaction causes you to grab the weapon with your stronghand, fine. A disabling blow to the throat can be delivered weak handed. The main thing is accuracy and followup.
If they don't buckle and/or fall, do it again. If they grab their throat and do not fall maintain your grasp on the weapon, direct it skyward and pummel them with your fist until they let go of it.
Grappling with an armed nut is very risky. Unless you can overpower them you have a tiger by the tail.
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Je Suis Prest