1. too easy
2. it will cause lag.
3. difficulty to shoot is what balances the guns. An obvious as hell aiming arc silhouette is just insulting.
4. a simple fix on the aiming notches to make it clearer wtf the notches mean will serve as a far better way to help (without breaking the game).
range finder tells the range in numbers. then you look for that number on the aiming notch on the gun. voila same solution. less obnoxious and far more simple to implement.
Of course it would be easy. Making shooting easier is the whole point of the rangefinder. Assuming the thing could function properly that's effectively what it's already supposed to do. All I'm suggesting is one method of making the information it's trying to provide easier to read.
As far as "it will cause lag" I highly doubt you know that for sure, but that's not a concern in my mind, as I'm trying to think of functions that would make the rangefinder useful assuming they function. I don't know if you think a shooting line would appear for everyone on the ship, but in my mind it would appear only for the person on the gun. Like the circles currently do, and the shot path currently implemented in practice mode does.
"difficulty to shoot is what balances the guns" is simply not true. When planning a strategy of attack on an enemy ship it's always safer to assume they can hit every shot, and when thinking tactically "oh their gun is difficult so I'm fine" is a mistake likely to end in death against experienced crews or even talented low levels. Plus saying "oh no don't use that gun your crew's not good enough" is a great way to make people not want to play the game, and corrals people that already play the game away from those guns. Not only that, but dedicating a pilot slot and taking time to watch people with the binoculars would create it's own balance dynamic. For instance pilots may ask themselves "if I stay back and the ally protects me can I sacrifice the slot to help accuracy?"
And for clarification on what the notches mean, I'm assuming you mean the notches on the lumberjack and hades? Assuming I'm understanding what you're suggesting here, that solution would make the rangefinder pointless for any gun that isn't modeled with notches. It's not like the Lumberjack and Hades are the only guns that are used at long range. For instance, the heavy flak. Not only that, but just telling the range is pretty much useless since one could just open the map and estimate distance from there. Range-finder would still be obsolete since people would say "No need to waste the spyglass or pilot tool slot, just look at the map."
Basically I think a tool intended to help gunners figure out their shots is an interesting idea, it was just implemented insanely poorly. People that don't know any better yet try to take it thinking they can help their gunners aim. That in itself isn't a bad thing in the right hands, but due to a confusing and distracting indicator that doesn't even properly calculate lead half the time it became counter-intuitive and harmful as it wasn't able to pull off what it was supposed to do. What I'm suggesting is a way to make the range-finder clearer so it properly pulls off that function of helping a gunner aim. Adding a simple shot-indicator graphic is intended to replace the vague circle things and portray information in a way that helps the gunner more accurately predict where their shot is going to go, as well as remove the need for the system to make the lead and drop calculations that it currently screws up.