The Air Service models were stripped of the rest of the military hardware; cut down stock, no upper handguard, bands, etc. No sling. Shooter just used the battle sights. 

Imagine being an observer in a balloon filled with hydrogen, where your real job is telegraphing information to artillery batteries.  Along comes the Hun in a maneuverable airplane flying at over 100 mph, looking for balloons as juicy targets.  The Hun has one or two machine guns, belt-fed, loaded with tracers and incendiaries, trying like crazy to ignite the hydrogen just over your head.  You get an extra 20 shots in the magazine of your 03.  Do you have a parachute?  Or will you hope to slide down the line that tethers the balloon before the damned thing falls burning upon you.