Officially known as "Machine Gun, 7.62mm, M240", the M240 and its variants are based on, and except for some parts on some variants, parts-compatible with, the Fabrique Nationale MAG 58. This is advantageous in that many other countries use the MAG 58, allowing for ready logistical support between NATO allies.
The current most widely-used variant is the M240B, adopted after a trial in the 1990s against a late-model M60 variant. Following the success in the trial, the M240B was designated as the infantry variant, having a stock, accessory rail, and other minor improvements over the then exclusively vehicle mounted M240.
It has been in service with the US Military in various roles since the 1980s, and has been rated as one of the most-liked weapons of the US Army infantry, with consistently high scores in reliability, and an unprecedented 100% confidence rating. Keep in mind, these scores were composited from surveys, completed in 2006, given to over 3,000 US Army, Guard, and Reserve soldiers who had used the M240.
Some criticisms, which are currently being addressed in the form of improved variants and parts kits, were overall weight, heat shield installation, and ammunition carrying solutions. In particular, the bipod has faced serious criticism as too heavy and not durable enough.
There have also been reported issues with the logistics required to obtain crucial spare parts, with upwards of 40% of troops saying that they had had some amount of difficulty obtaining things such as barrels, springs, and pins.
The M240B can often be found equipped with either a C79 or ACOG optical sight, as well as a laser designator, and some special purpose M240's have been seen with a suppressor attached.
Fabrique Nationale brochure
US Army info on the newest infantry variant, M240L