Hadn't noticed this thread until now, for some reason...
Snipers have a range that is effectively infinite. Actually it's 999,999,000 but since I don't think it's possible within the game for anything to get that far from anything else, treat them as infinite.
The game treats them as truly infinite, actually, the range value is just north of a special threshold value beyond which it doesn't even bother doing range checks or any of that sort of thing.
And snipers (turret or otherwise) can still be useful under an ff depending on your circumstances. If the enemy has a huge sniper presence on a world, "under an ff" might be the only place such a unit could last
Also, if you really need to cover them without the damage reduction, a Riot Control Starship's shields can cover nearby units and don't apply the reduction. The tradeoff is that the Riot's shields are waaaay weaker than a full-blow ff generator. Shield Bearers and Spirecraft Shieldbearers are also useful for this, if you have access to them.