The smg

The M7/Caseless Submachine Gun is an automatic UNSC submachine gun that fires from a 60-round magazine which is placed horizontally on the left side of the weapon. Due to the odd horizontal placement of the magazine on the weapon, it can be assumed that the magazine utilizes a circular ramp where the magazine meets the breech. This rotates the horizontally stacked rounds in the magazine 90 degrees until they align with the horizontal breech. An example of this can be seen in the present day Fabrique Nationale P90 SMG, as well as the HK G11 experimental assault rifle, both of which utilized a vertical loading system. The HK G11 also used caseless rounds like the M7. It also has various points which it resembles the Heckler and Koch MP7, such as the name, foregrip, frame of the weapon, retracting stock, etc.

The SMG has a polymer handle, folding fore-grip, iron sights, and retractable stock, as well as a titanium body, the SMG must be cocked before it can fire the first round. The charging handle is located on the right side of the gun and does not move during operation. Once the first round is fired, the gases from previous rounds force the breech to rotate and chamber a new round. Once the magazine is empty, the handle although not illustrated in-game can either be pulled back and locked or it can be fully cycled after a fresh magazine has been housed. If it is first pulled back and locked, then it must be pushed forward in order to chamber a new round. The magazine release button is located between the red dots on the receiver and needs to be pressed before it can be flicked off. There is no ejection port due to the nature of the rounds fired.I have got this info from halopedia.com