1200 is their base range and you add 500 to that to get the range = 1700
(ignoring tech bonuses)
This means that before the bowmen can send a volley at the defenders they have to travel 500 across the battle field. If their techs aren't high enough to travel that 500 they won't get to attack in their first turn.... they'll just move forward.
Bowmen have a base speed of 250 so in order for them to cross the field distance they need an extra 100% bonus to their speed. RD and Dragonry add 5% per level so by that logic you'd need to have RD and Dragonry at LVL 10 in order for bowmen to attack in their first turn.
I don't have Dragonry and RD both leveled up to 10 in any of my games so I can't experiment to prove this. There may be some evidence in the Dragamomicon that supports this theory. I'll have a look.
If this is the case, the true offensive potential of LBM isn't really released until you have both these researches maxed.
Cool, look forward to analysing one myself!