Bayonets epitomize the warrior mentality. Although it's been a good while since the last official call was made to "fix bayonets" in an actual combat mission, the ancillary CQC weapon retains a special place in many warfighters' hearts. Of course, if troops like to attach a sharp, pointy knife to their rifle's end, then they'd surely love to affix a chainsaw. What could be better?
Chainsaw bayonets have become a trope in popular sci-fi, but there is none more iconic, overly-gratuitous, and awesome than those attached to the Mark 2 Lancer Assault Rifle in the Gears of War series. This futuristic weapon is a massive, fully-automatic rifle outfitted with a roaring chainsaw bayonet. It works well in the game, but it wouldn't stand a chance in the real world.
There aren't any official technical specs available for the Lancer, so it's impossible for us to accurately judge its effectiveness, but we've seen a few people try to recreate the chainsaw bayonet themselves. Still, this technique is nowhere near as common as pop sci-fi would have you believe — for good reason.
In real life, the chainsaw bayonet is extremely flawed for a number of reasons. Firstly, there isn't really any way to store the gasoline needed to power the chainsaw, so it won't run for long. The workaround here would be to add a larger fuel source, but by doing so, you'd add to the already-bulky weight of the saw.
Then there's the weight-distribution problem. It's never an issue for the hulking heroes of Gears of War, but real-world troops aren't so massive. Adding weight to a rifle will likely throw off its center of balance. When the front of a gun is far heavier than the back, it simply won't fire accurately.
The center of balance is almost always closer to the butt-stock so the user has more control over control the weapon. Firearms without butt-stocks are also balanced in a way so that the recoil doesn't shift the sight picture. Attachments to the front of a weapon, like suppressors, can help regulate weight distribution, but these are very specialized tools. The bulk of a functioning chainsaw would be incredibly difficult to offset.
Finally, we have a hard time seeing a situation in which a chainsaw bayonet would be more effective — not just more enjoyable — than a standard bayonet.
For a quick rundown on why this weapon would also be a complete safety hazard, check out this video.