Chainsaws are very useful tools in many situations, but in science-fiction, they are generally shown in a very different role. From the chainswords and chainaxes used throughout Warhammer 40,000 to the chainsaw bayonets from the Gears of War series, chainsaws in science-fiction are more often shown as weapons than tools. Since they are used in this way throughout fiction, it might surprise some people that chainsaws are not used in combat by any world military; however, there are reasons for this. Unlike many of the science-fiction technologies I have discussed before, the technology to create chainsaws is well understood and the results are very effective. The reasons they are confined to use as tools are based on more practical concerns.
One of the biggest reasons that chainsaws are not seen in combat is simply that most combat now happens outside of melee range. A soldier armed with a chainsaw would be an easy target for an enemy with any type of ranged weapon, which is the same reason that bayonets are only issued as weapons of last resort. Even as a back-up weapon, a chainsaw has many practical disadvantages over something like a bayonet. A chainsaw is heavy and loud, meaning that someone wielding one as a weapon would be more obvious and considerably slower than someone with a bayonet or knife. Since stealth and speed are vital on the battlefield, these are considerable disadvantages. Another serious drawback is that chainsaws require fuel, which increases weight, throws off balance and creates the possibility of a soldier being left with what is essentially a very awkward metal paddle if the fuel runs out.
Besides the physical problems with carrying a chainsaw into battle, there is also the issue of the danger it presents to the user. A chainsaw is not a safe tool: the cutting edge is a toothed chain that rotates at very high speeds, and if used in combat it would be almost as dangerous for the user as it would be for the enemy. People who use chainsaws in their professions, such as lumberjacks, are slowly and carefully sawing through stationary targets, while using a chainsaw in combat means swinging it around very quickly at a moving target. The potential for the user to accidentally catch himself on the blade is very high. This could be alleviated somewhat by placing a guard over half the blade, but that would also mean half the weapon could not effectively be used. Swinging a chainsaw around would also increase the risk of kickback, a phenomenon where a chainsaw pushes backwards towards its operator. This could obviously lead to disaster. Even assuming everything went right for the user, a chainsaw requires being pressed firmly against the target in order to cut properly, and so the type of quick slashes used in combat would not provide the spectacular results usually seen in fiction.
Chainsaws are also not as effective as one might think on the battlefield because of what they would be used against. Even wet wood or sap can clog up a chainsaw’s chain and prevent it from working, so using it on a wet and muddy battlefield against a soft target would be a recipe for disaster. Using a chainsaw against a hard target such as an armored vehicle, would be even worse because of the risk of the chain breaking. A chainsaw chain is rotating at very high speed, and so when it breaks it lashes out, and likely strikes the operator. Chainsaws are designed to cut through wood, not metal, and so unless special chains and motors were designed, using them as such would likely be disastrous.
On the whole, chainsaws are dangerous and temperamental devices that are impractical for use on the battlefield in any capacity beyond combat engineering roles. Despite how they are portrayed in fiction, chainsaws are not a super-weapon capable of defeating any foe. In fact, such a weapon would be just as likely to maim or kill the person using it as his enemies.