One of the many things I learned while working in film over the years was the ergonomics of weapon design, because we had to design and build so many of them practically for the films I worked on. Some of the basic rules for gun design are illustrated on this image and explained below…
A stock considerations
The line of force from the bullet firing out of the barrel is transmitted through to the stock, enabling the recoil to be absorbed while keeping the gun straight for firing multiple rounds. I see many gun designs where the stock is angled down well below the line or in other odd positions. Firing the gun for real would cause it to fly right out of the user’s hands or hit them in the face!
b what’s your bullet shap e?
Think about the bullets and how they fit into the magazine. Banana magazines seen on AK-47s and on this ACR curve to the front because the shape of the bullet tapers toward the front. This forms an arc when stacked together, so that the bullet at the top of the arc will shoot straight out of the barrel.
C get a grip
The relationship between the hand/ trigger and shoulder shouldn’t be messed with. All fixed stock weapons will maintain a similar proportion: this gun has an adjustable stock, but the range of variability still stays within a fixed proportion for human use.
D eject, eject!
There has to be at least the length of the bullet behind the shell ejection port to allow room for the mechanism to fire the bullet. This applies to all traditional weapon configurations, although newer systems permit this space to be slightly shorter, as seen in the KRISS submachine gun.
E take account of design tweaks
There are some exceptions to this straight line rule. Some variations of the AK-47 have a stock that’s angled down below the line. But this gun also has a special muzzle break up at the front to counteract that aspect of its design. So if you change something in your design, introduce something else to balance it out.
F don’t break the laws of physics
Make sure there’s a clear line from where the shell ejection port is to where the bullet is fired out of. I often see guns in games where a bullet has to break the laws of physics, go outside of the gun, or rotate 90 degrees to travel from the magazine and out of the barrel. Even if the audience knows nothing about guns, they’ll realise something’s off and it’ll take them out of the experience.
Firing the gun for real would cause it to fly right out of the user’s hands or hit them in the face