Not sure if I'm late to the party on this one, but /checkparam confirms that Stabilizer and Stabilizer II do in fact affect ranged accuracy!
On another note, Scope nearly negates the effects of Drum Magazine with few maneuvers up. Here's a quick and dirty test:
No attachments, no maneuvers: 727
Just D.Magazine: 710
Both, by # of maneuvers:
- 0 : 722 [-5]
- 1 : 722 [-5]
- 2 : 711 [-16]
- 3 : 700 [-27]