G&P's Ball Rifle AEG is a carbine length M4, something one would use in a close quarters battle (CQB) situations to mid-range engagements. The carbine length restricts you slightly in a tight environment more than makes up for it once you start going from room to room to an outdoor play style.
G&P have released a M4 Quad Rail System allowing room for ample amounts of accessories ranging from scopes, sights, grips, panels, etc. Using it as-is, its low-to-mid power makes it appropriate for indoor use. You can compliment the weapon further by using only low and mid cap magazines to avoid the slow-winding nature of hi-caps. You can customize the ergonomics to better suit your tastes; you can swap out the furniture for colored variants and add the same color rail covers to the front end to dual-tone your weapon.