@Sweet_O:
In OCTH mode, pressing 'F' hotkey will display your actual chance to hit.

In NCTH mode, pressing 'F' hotkey displays only your muzzle stability which is simply the mercs ability to stabilize and focus on the target. There are a couple other factors that go into determining actual chance to hit that are not displayed on the 'F' hotkey. These are represented by the size (and color) of the cursor aperture.

As far as sniping goes, alot of your issue is probably scope related. NCTH was originally designed with some pretty extreme minimum effective ranges for scopes. And using a scope within it's minimum effective range results in a progressively worse muzzle stability penalty. There are two settings in CTHConstants.ini you can tweek to alter the effects of minimum effective scope range:
SCOPE_RANGE_MULTIPLIER alters the actual minimum effective range directly. Minimum Effective Range is NORMAL_SHOOTING_DISTANCE * ScopeMagFactor * SCOPE_RANGE_MULTIPLIER. In other words, the default minimum effective range for a Sniper Scope is 490units (or 49 tiles). Using a sniper scope against a target closer then 49 tiles will result in a muzzle stability penalty.
AIM_TOO_CLOSE_SCOPE is used to calculate the actual penalty applied. AIM_TOO_CLOSE_SCOPE * 1/2 * ScopeMagFactor is the maximum penalty. So the default maximum penalty for a sniper scope is -20. This may not seem like alot, but it really is.
I should point out that the penaly applied is based on range. A sniper scope used at 48 tiles will have a much lower penalty then a sniper scope used at 2 tiles.