Basic Assault Weapon *updated with inside pictures for building*
This is a basic weapon that is mostly compact and pretty reliable. There are no big special features other than the magazine which is obviously removable and has an internal pusher, a roof, and it can be top loaded. With a quick change of pieces you could technically top load the magazine while it is in the gun though it may be a little tricky. The magazines slide right in and should stay put. I shaked it my hardest and it wouldn't fall out. Removing it is pretty simple. You just need to pop both sides individually.
As you can see I ran low on pieces so I just used red rods for the stock. You can store an extra magazine in it like in the picture. Of course I put in a pin guide. The handle is average. I wasn't too concerned with it as people can mod it freely. The trigger was made to block farther back so that the gun wouldn't have to be a unit longer to get max power. Ranges with the rubber bands placed as shown were about 9 meters or 27 feet with the mag full. I suspect increased ranges as the magazine empties. It doesn't have a turret, bolt, or shells so I don't know if you consider that good enough or not.