I believe the stock Sniper Rifle can be fired while holding stuff in the off-Hand.
In regards to the penalty, i have to say i didnt manage to check how big the Penalty for holding stuff in the off-Hand actually is.
However, it is my impression that firing a Machine Gun while holding a Mace! should induce a severe Penalty, surely not only 5% or 6%. But more like 20 % or more.
In regards to the Sniper Rifle, a potential Penalty should be considerable. In fact, a sniper rifle, in my opinion, should probably be true 2-handed.
Also, in my opinion, even a Shotgun should not be able to be fired 1-handed without a Penalty, though this one should be only small.
I believe the only weapons that should be able to be considered true 1-one handed should be pistols. Even true SMG (i.e. Blackmarsh) should have a small (10?) Penalty, while a UZI or Scorp should be 1handed.
However, of coure this is only my opinion and if Diox and/or you dont agree or deem is not worth the hassle, so be it.