Maybe when aquariums become a thing, and that probably won't be until aquatoids start showing up for real instead of just being a Bootypedia article.
Funny thing is, when you dont have a Burrow at a base the error message for capturing or sending animals to it references an Aquarium.
All shots done with 200% hit-chance into smoke/without LoS always hit unless the bullet goes 'through' or 'over' the hitbox.
Re: lancev
Percent chance to hit is only as it says on the tin when it comes to melee attacks since they dont involve a trajectory. Someday melee attacks may be converted into projectiles with a clause (melee attack roll) to deny an impact on a targeted unit, but that day is not here yet.
When you shoot at something the % chance refers to the cone of possible trajectories, and it represents something similar to MOA in rifle shooting terminology. For example 1 MOA is 1" possible dispersion at 100 yards (which would grow to 2" at 200, or 10" theoretically at 1000 yards, this is used partially to determine the maximum effective range of firearms IRL).
100% chance to hit is a guestimate from the original designers that you would not be able to miss a target (Man Sized presumably) at a given number of tiles. Probably 10 or 15 tiles since most engagements in vanilla xcom occur either up close, or at 10-20 tiles.
Also, while the shot cone may say that you have a high chance to hit the target, the Perfect Trajectory having already been calculated to see if you have a Line Of Fire... it will not necessarily follow that Perfect Trajectory even if you would still hit that target guaranteed. Which means if there is any obstruction between you and the target such as a Fence or Window or the edge of a Bush the shot may hit that instead.
(this is also why force firing on a target can be beneficial since the perfect trajectory may clip some scenery object, but other trajectories may still hit that target and miss that object... besides the capability to just shoot the offending bush and then fire again after it crumbled)
Its happened lots of times to me both in OXC and in vanilla xcom where I really have to make that shot, I get the guy with the high accuracy who's batting something like 120% even on a snap shot... fire from some considerable distance and the shot hits a damn bush.
Edit - Just figured id answer this while it occurred to me... while its easy to say that an assault rifle has an effective range of at least 300 yards (=tiles) the aiming skills in xcom dont concern themselves with ideal situations and shooting from a bench at a target range. Or shooting at a group of soldiers casually walking down a road from a hidden position.
All units are assumed to be moving around, including your own, and when they see an enemy they only have a narrow window of time to acquire target and fire. Any number of factors could impact the accuracy of that attack... such as the enemy's movement (prior to the position they ended up at), hesitation on the part of the shooter, or even the shooter trying to maintain an awareness of their situation outside of the target they're focusing on.
(Even when the target has panicked they could be spazzing out, spinning around, and otherwise moving all over their 1 tile of confined real estate, so they are not stationary and an attack roll is still valid)Snap shots are literally pointing the gun in the general direction of the target and hoping to get lucky. Aimed shots mean trying to hastily line up the sights on the target while compensating for their movement (to lead the target), maybe spending about 1 second on it, nothing like we would consider a truly sniper-esque aimed shot to be.
(with Sniper Rifles an aimed shot is probably like it is in FPS video games where you try to shoot someone quick so its kinda a ballet of crosshair movement, and/or trying to ambush the target with the crosshair and time it exactly - I be starting to think that Reactions should have some role in shooting sniper rifles because of that, since using a scope makes actually targeting something easy peasy and quite abstract)
And all of this reminds me of another topic that may be controvercial, that each Tile is actually probably more indicative of a 5x5 foot square instead of measuring yards or meters. Mostly because of the scale of buildings and scenery and the destructive radius of lower end vanilla explosives. There is also vertical height/floors to consider too.
