M4s became popular due to their diverse upgrades and the simple fact that they are extremely adaptable. M4s can be custom built to be used in nearly any environment. The upper receiver can be swapped out for a longer or shorter barrel in seconds, the stock can be swapped out for larger, smaller, more adjustable stocks, or even no stocks. The mags are extremely common and can be found with a high range of round capacity all the way from real capacity mags at 30 rounds all the way up to box mags at 5000 rounds. Typically, they have a large amount of rail segments that can be used to add attachments such as scopes, red dots, lasers, flashlights, bipods, foregrips, grenade launchers, etc. M4s can be quickly, and easily adapted to their users preference.