Obviously I cant speak for Bungie however here are a few factors which might have influenced their decision. The vast majority of bullpup designed weapons are assault rifles, the main reason behind this is to shorten the length of the weapon, increasing their maneuverability in close quarters. As its not typically a close quarters weapon having a shorter sniper rifle isnt necessarily that advantageous. There are a few famous bullpup sniper rifles the Styer SSG and the Barrett M95 etc (they tend to be the exception not the norm though) The down side of bullpup designs is that the mechanism tends to be more complex (more complexity means a greater chance for stoppages) Another serious disadvantage is the fact that the magazine is a few inches closer to the the snipers face, if you were unlucky enough to have a catastrophic failure instead of the explosion happening 3-6 inches away from their face it happens right by their cheek/eye. (Catastrophic failures- are usually caused when a defective case is struck by the firing pin, instead of the case containing the explosive gasses and propelling the projectile down the barrel they blow backwards, damaging the bolt and blowing the magazine off the weapon) And also I guess perhaps from a design perspective they wanted to have a bit of variety to the designs. I have a concept art book for the original Halo at home somewhere and as I recall the artist who designed all the weapons (I cant remember his name) was extremely technically minded when it came to weapons. To the point that when he saw that one of the animators created a new animation cycle for when the player was AFK, Chief would start fiddling with the scopes wind age and elevation knobs, apparently he freaked the f**k out. A sniper would never do that in the field. hope that helps.