Are there any rules for this?

The size modifiers give examples of a knife as a Miniscule target, so obviously you should be able to shoot at targets of this size. But what happens if you do hit your opponents gun? Does it break? Does it in effect disarm him? Do weapons also have APs and wounds?

I searched Core and IH, and far as I could tell was that lathe upgrade gives your weapon Armor equal to 25.
nowhere was rules of breaking items, their armors, wounds, nothing. or I just missed them.

PnPGamer is correct!

At best, you can apply the rules to breaking Cover to weapons and items, but you'd have to come up with what the items are made off yourself.

I'd say shooting someone's gun though would cause it to fly off- then again thats depending what you shoot it with. A Bolter would cause it, and the hand, to blow up, whereas a regular bullet may just send the gun flying.

And a krak grenade to destroy it outright?