I've had the Huntsman for a while, and it's still confused me. The projectiles seem to be affected by gravity, most definitely. But sometimes, when I stretch out the bow, the arrow seems to barely make an arc (meaning straight line), while other times, it seems to just make an arc.
There's also a bar that's shown as you pull the string back on the bow, what is this used for?
You need to pull back for 1 second to get a Full Length arrow off. If you pull back for more than 5 seconds, your hand starts to wobble and you will lose accuracy (I'm guessing this is what you are experiencing when you say that sometimes it doesn't go in a straight line)
So in order to get a good shot off, you need to charge it for 1 second (which is what the charging bar shown is) and fire the arrow before it starts wobbling (which occurs 5 seconds after being fully charged)
If you find yourself with a charged shot longer than 5 seconds for whatever reason, you can switch out your weapon to reset it, or better yet, press right click again to 'unload' the shot (does not waste ammo)
No matter how you shoot it, the arrow is always affected by gravity, but even more so if you do not fully charge the shot (from memory the shot does less damage for shorter charge as well) so make sure you always charge for at least 1 second
The two things you have mentioned (variable arrow arc, bar shown as you pull string) are connected. The bar you see is the charge bar, same as with any sniper rifle. Pulling back the string by holding down primary-fire is your charge method, as opposed to rifles where you click once to zoom in. At no charge, arrows arc significantly and move rather slowly. At max charge, arrows fly almost straight much faster and do 2.4x damage. Arrows will headshot at any charge unless you let your aim get wobbly; you can reset the wobble with alt-fire.
