So there used to be this ambiguous rule:
You can bash an opponent with a shield, using it as an off-hand weapon.
As gatherer818 reports, the bolded section was removed in an errata. This is good, as it clarifies how the rule works. However, despite this statement, you could use a heavy shield two-handed even before the errata. Here’s why:
There is no such thing as an “off-hand weapon” under the general rules
The two-weapon fighting special combat option refers to “attacks made with the off hand” which is the closest the rules ever come to using the term. Even there, it is in a specific case, not a general purpose rule.
If you are not using the two-weapon fighting special combat option, you do not have an “off hand.” It is not a general term applied in the rules.
As such, the phrase “as an off-hand weapon” is descriptive
The rules text for Pathfinder is a mix of description – statements which illustrate how things are used – and proscription – rules, requirements, and limitations on actions. It can sometimes be difficult to tell the two apart; this is something that some rules systems (D&D 4E, Legend) endeavor to correct, while others (most notably rules-light systems) embrace by doing away with proscription altogether, but there are nonetheless a lot of systems, like Pathfinder, that use a mix of the two.
Since “off-hand weapon” is not a game-term, and the phrase is used in an introductory sentence for a special combat option, it is being used descriptively – how the author expects you’ll want to use it. The original author of the line, which also existed in 3.5, even explicitly stated this fact. (link pending)
Attempting to read the line proscriptively makes no sense
Both from game balance and verisimilitude perspectives, claiming that a shield can never be a primary weapon makes no sense. This has nothing to do with two-handing the shield at this point, it’s about using the shield as a one-handed weapon, rather than forcing it to be the light one (or worse, a one-handed weapon used as the off hand attack in two-weapon fighting, accruing substantial penalties)
Game Balance
Heavy shields are one-handed weapons when used to shield bash. As such, they have to be the primary weapon, or else you take large penalties in the one case where “off hand” is even mentioned in the rules – two-weapon fighting. To effectively use a heavy shield as a weapon while dual-wielding, you must be using it as your main hand, and a light weapon as your other weapon.
Verisimilitude
Which perfectly describes the combat style of the Roman legions. Seriously, the primary melee armament of Roman legionaires was a large, heavy shield, and a small, stout gladius – a short sword. The style is more effective when you have an organized formation, of course, but that doesn’t mean it suddenly become physically impossible without that. A Roman legionaire cut off from the legion and forced to fight on his own would still fight that way, because guess what – those are the weapons he’s got, and that’s the style he’s been trained in. And while it might not be ideal, it’ll still work pretty well.
Shields make fine bashing implements, particularly when they’re heavy, which they often are. Their reach leaves something to be desired, and it can make it difficult to use the shield for its primary purpose, but it still works just fine.
Two-handing the shield is therefore allowed under the rules, and also can make sense
Imagine a big, heavy shield strapped to your left arm. You can hold it up to block things, swat at guys a bit with it, but what do you do if you really want to bash someone with it, say if your primary weapon’s been knocked out of your hand?
You’d grab your left fist or wrist with your right hand, and swing with both together, putting your whole weight behind the blow. Certainly seems to justify tacking half your Strength onto the damage to me!
And I’m certain there is ample illustration of this technique from Captain America comics or movies; I’ll try to dig those up tomorrow when I get a chance. For that matter, consider Captain America: he usually fights with a shield on his arm, and no other weapon. He’ll punch with his fist, certainly, but he also fights with the shield. It’s iconic. His fighting style is often very much shield as primary weapon, fist as the off-hand, secondary weapon. Sometimes putting a little more oomph into the shield bash is worth not getting to punch.
Best Answer
I would simply use Dark Ages rules. In Dark Ages a shield possibly:
For slow projectiles, such as thrown stones, or if you have Celerity, you could probably even try to actively parry a projectile, with Quickened Senses combo discipline (Auspex 1, Celerity 3) you could probably even parry a bullet.
Ballistic shields are relatively heavy, so I would make actively parrying with them (if you don't have another weapon) even harder (+1/+2 to Difficulty) unless a shielder has a sum of Strength and Potence of at least 4.
I would agree with Space Ostrich in that shields can be treated as cover against missiles, as listed on page 278 of V20 Corebook. Dark Ages basically offers the same thing (increases difficulty to hit you).
Shields, even ballistic, differ in size. Some only cover your torso, some can be treated as "only head exposed" (+3 difficulty to hit for the opponent) if not as +4, as some shields don't even expose your head. Keep in mind that 10 is always a success result anyway. However, because ballistic shields are especially designed as cover and normally have designated observation slits and firing holes, I wouldn't add any penalty to the shielder (just like using a shield in Dark Ages). If the shield covers you fully and you are not shooting at the moment, hitting you from the front would be totally impossible.
As one of your hands is occupied, you can't use a weapon that would require using both hands, such as a sniper rifle, only a pistol or a small SMG would be OK. Or a katana. :3
Ballistic shields are a very resilient device, I didn't happen to find a test video which would show some weapon's ability to penetrate it, even AP 12-gauge bullets didn't succeed. I would assume that some very, very heavy guns like stationary high-caliber machine guns would penetrate them, but this is nothing more than an assumption.
Riot shields, on the other hand, are light-weight and don't always offer protection from firearms (though, some protect their user from handguns and ordinary shotgun slugs). If a shield is hit, I would simply add several dice to target's soak pool depending on shield's resilience, but make parrying difficulty relatively low, they are as easy to use for melee combat as medieval shields if not better. The same story with soak dice would go if trying to penetrate a ballistic shield.
A riot shield is something like pavise shield of modern days, so I would apply the same rules for both. Sadly, I didn't manage to find the stats of pavise shields.