Let's say I have a MCT Roto-Drone and I want to install a Pop-Up mount on the body for an AK-97.
By the CRB pg 466 under Roto-Drone, you treat the body as 3 higher for the purposes of weapon mounts and customization. By Rigger 5 pg 122, you have Modification Point (MP) equal to the drone Body attribute, which is normally 4 in the case of the Roto-Drone, but it is highly customizable so that number is actually 7. You have a pool of 7 MP to spend for the Roto-Drone.
Since an AK-97 is an Assault Rifle, consult the chart on Rigger 5 pg 124. We will need a Large Weapon Mount (12F) to mount the weapon, which will consume 4 MP. However, running around with a drone making serious heat will get the people looking and HTR on your case. Adding a blow off panel is not convenient, so we will add a housing and a pop-up mod to the mount. The pop-up mod will increase the MP cost of the Large Mount by 1, making it 5. The added cost will be 100 x New MP Cost (50) to the base price of the mount.