From Worms Knowledge Base
This weapon is placed like a mine (well, almost) and is similar to the Beehive in principle. And to some extent, it acts like one. Fitted with uzi-like cannons and an infrared sensor, it is designed to shoot at any worm that moves in its line of sight, whether it is that worm's turn or not.
The worm with the initial sinister look on his face attaches the device to the landscape depending on the slope and suitability of the landscape. If it's a pixel that the worm is trying to attach it to, the all-too-often heard 'girder-beep' will announce it's objection to the attempt. If however, it's a fairly straight piece of land, either flat or at a slope (even a steep one), then the girder-beep is nowhere to be heard and the device latches onto the terrain.
The worm retreats to a safe distance. When the turn ends, the device activates. From that point on, any worm which makes a noticable movement, (e.g. walking) will be fired upon by the device with an array of uzi-like bullets. The Sentry Gun cannot fire in all directions, but only the 180 degrees defined by it's orientation on the landscape. (i.e. if on flat ground, it can never fire downwards)
The Sentry Gun can be blown up, like a barrel, if a worm manages to shoot at it. The Sentry Gun has infinite ammo, but does have weaknesses. It's turret is slow, allowing a player to move quickly through it's range and avoid being shot if the turret is pointing in the wrong direction. Each round of bullets fired by the Sentry Gun is seperated by a slight time gap, so a worm being fired upon is not doomed to being shot continuously until death.
The Sentry Gun would possibly be too powerful if it automatically shot worms just as any other weapon, so a variation involves the idea that worms do not lose their turn when shot, and therefore have a chance to escape. They lose the health at the time of being shot, however, rather than at the end of their turn. This comes close to Real Time worms, and so involves many technical problems, such as the Sentry Gun acting like another worm by firing uzi-like bullets during another worm's turn, and the ability for the stricken worm to not lose his turn after being shot at, and what happens if the worm is shot at while using a tool such as blowtorch or jetpack.
Another possible problem is that the terrain requirements for 'placing' the Sentry Gun would require extra code to detect the suitability of the terrain at which the worm is attempting to place it, and if it is suitable, then determine which orientation the Sentry Gun should have.
In a way, this weapon removes strategy from the game by letting something else do the shooting for you, and not even when it's your turn. But in another way the Sentry Gun can be a very useful tool in guarding areas of land and blocking off territories, particularly the entrance to tunnels. The weapon isn't too powerful either, as it not only shoots anyone, even the worms that placed it, but can also be destroyed easily with weapons that do not require the shooter to be in the line of sight of the Sentry Gun. It will still waste their turn though, and a well-placed Sentry Gun can be hard to shoot with any weapon.
If an unfortunate player starts his turn in the line of sight of a Sentry Gun, he will be unable to move without being shot at, unless he is quick before the weapon can aim at him. But he will be in the perfect position to destroy the Sentry Gun providing he has the right weapon available, or he could simply not move at all and shoot another player anyway, which would be especially more effective as that worm is also likely to be in range of the Sentry Gun, providing a greater damage opportunity and also a distraction, allowing the possibility of escape.
This weapon goes against the traditional turn-based strategy in some respect, but not much more than the land mine does. It's suitability to worms is debatable, but one thing is certain: the application of this weapon in offline missions should be more than enough to guarantee its acceptance.