How can my character who is missing a thumb use a handgun? Can some kind of handicap device to built? Or would you recommend using their non dominant hand that does have a thumb?
So, this kinda depends on a couple things. In general, the first suggestion would be to learn to shoot off hand. It’s easier, and you don’t lose much accuracy with practice.
Normally, first metacarpis creates a natural resting point for a handgun grip (and most firearms), allowing the weapon to transfer recoil into the user. If your character is completely missing their thumb (as in all three bones are missing), this resting point will be absent, and without some kind of careful accommodation (such as a completely custom grip, or surgery), it would probably be impossible to control recoil.
If the thumb had been lost as the result of trauma, it’s likely that the first metacarpal would still be intact, meaning their grip would be less secure, but recoil would be manageable. Again, using the off hand would probably be preferable, but it would be possible for them to use a firearm with the thumbless hand.
There’s an additional consideration here. Some firearms are not designed to be used in the left hand. This isn’t a unique issue for your character. How ambidextrous friendly a handgun is varies by the individual models, sometimes even within different generations of the same gun.
Generally speaking, there’s three tiers of ambidexterity in handguns. Firearms that cannot be reversed at all, ones that are normally ambidextrous, or mostly ambidextrous, and ones that can be easily converted depending on the hand.
What this means for you is, you may need to check the specific firearm you’re thinking of.
The issues to look at on a handgun are the grip, magazine release, slide release, and safety.
Asymmetric grips are a little unusual in handguns. They’re more common with sporting rifles. In cases like this, you’d need an entirely separate replacement grip to fit the off hand. In a few rare cases, it may not be possible to replace the grip at all.
Asymmetric magazine releases are more common. This includes things like the 1911, Glocks, Beretta 92, and some SIGs. These use a simple push button magazine release, which is mounted at the bottom of the trigger guard. It’s easily accessible with your thumb, while holding the firearm with your right hand. However, depending on your grip in your off hand, you may need to adjust the pistol significantly to kick the magazine out when holding the gun left handed.
Additionally, because these magazine releases rest under the middle finger while holding the gun with the left hand, it’s possible to accidentally drop the magazine when firing, due to recoil on some models, for some shooters.
There are several, semi-common, magazine release methods that are ambidextrous. A pair of levers located in line with the trigger guard, which can be pressed down to release the mag. Sometimes, instead of a single push button, there will be one on either side of the frame. These usually will work if either button is pressed. Finally, an older style is a simple mechanical catch at the base of the grip which holds the magazine in place. Pressing this back will allow the user to reload. This last variation poses a unique challenge to your character, because, you press the catch back with your thumb, then pull down on the magazine with one of your other fingers, without a thumb, it would be significantly harder to reload a firearm that uses this style of magazine release.
To be fair, reloading may pose a unique challenge to your character, as you use your thumb to manipulate the magazine. This might be less significant if they’re simply discarding partial and spent magazines, but that’s expensive.
The slide release is a lever or button which will allow the slide to close after it’s been locked open. Usually, it will lock open after cycling with an empty magazine. This significantly speeds up reloading on an empty firearm, and provides useful information to the user that the gun is dry. In many cases, this is mounted along the slide, and can be accessed by depressing it with your thumb. However, if you’re holding the firearm in your left hand, you may need to reach over the slide to close it and cycle the first round into battery.
Swapping the slide release over to the opposite side is sometimes possible, but requires the user to replace the slide, and release lever in most cases. Now, some manufacturers do release entire kits, or mirrored versions of their pistols for left handed shooters. Though, fully mirrored weapons are something of an oddity.
It’s also worth noting that, replacing the slide will sometimes also reverse the ejection port. This isn’t a huge thing most of the time, but can make the gun more comfortable for a left-handed shooter, as the brass will be ejected away from their face rather than towards it.
Also, as a bit of random trivia: Many first person shooters feature reversed ejection ports on their firearms, so that the gun ejects brass in front of the user. There’s no technical reason for this, it’s done to make the gun more mechanically interesting when fired.
Ambidextrous safeties are more common, but it’s entirely possible a left-handed shooter will have to reach over the weapon to adjust these controls.
Now, I’ve been saying left-handed, on the assumption that your character is right handed. If your character is left handed, it will probably be easier for them to operate a firearm with their right hand, and stabilize with their left. Reloading would still be a challenge, however, simply because of the size, and weight of the magazines.
So, it depends on the exact condition of your character’s hand. If they’re right handed, and only missing a digit or two from their thumb, it might still be easier to use it in that hand. If they’re left handed, firing right-handed is probably the way to go. If they’re right handed, and the thumb is completely missing, down to the wrist, then the left hand is probably the way to go. Though, it is possible there might be some kind of custom wrist locking grip, I’m unaware of, that would allow them to operate it in their dominant hand without issue.