The problem is that the "rule of 13" just dissipates random magical energies. So it'd still be able to be hexed just as easily. Also, it wouldn't have any effect on GM-induced compels to a practitioner's high concept.
On top of that it wasn't just get sets of 13 of something and you're done. IIRC the pillars and tables were placed at just the right spots in the middle of the room, which did more than just interrupt random magic; it interrupted the normal working function of the room itself by breaking up the normal open lines of sight and forcing people to wind their way through them to move across it. Just imagine that place completely full and what would happen if everyone got up at once and started a panicked rush for the exit.
So get a look at the internal workings of a gun and try to imagine what you'd have to do to put 13 extra bolts through the internal mechanism and still have it function. I see the gun getting so complex that the merest physical jolt wouldn't jam something, much less the stress of firing it even once. In addition, the design of the pub didn't prevent any magic from hitting the pub, it just prevented it from hitting anyone else. Don't think of the gun as the pub, it's actually one of the customers; the protection needs to be separate from the gun and around it, like a Faraday cage to shield an electronic device from EMP.
Given Dresden's comment that random hexing mostly affects post WW2 stuff, I'd say that most service guns (as opposed to tricked out competition only "race guns") are already nearly immune from the effect. The Colt 1911 design is already 100 years old and at least one modern automatic manufacturer made a design at least as rugged and reliable. I'll avoid the tangent by leaving them out of the discussion, but I know from personal experience that one of my own weapons has exceeded the Colt 1911's original trial 6,000 rounds without a malfunction by more than 4,000 rounds. This isn't limited to handguns either, the venerable AK-47 design is a 65 year old design with a world wide reputation for reliability in the face of extreme adversity; cold, heat, mud, dust, soldiers too inexperienced to even learn how to clean the weapon, it just keeps going. The main heavy machinegun for the US military is over 90 years old; there just isn't much need to replace something that works as well as these weapons do, at least for personal weapons. You can't really get much more damage than dead on a human target and adding ever increasing layers of metal armor to stop bullets just isn't really an option the way it is for armored vehicles.
As for using computers, it should be doable. You have the computer itself in a separate room, which you don't enter, with just the monitor in the room you will be working in. The wizard gets comfy, drops a circle around himself and uses a wireless keyboard to activate the computer and turn on the monitor; which he can no longer hex from inside the circle. The computer ever loses contact with a keyboard for more than a few seconds and it shuts the monitor off. It's going to be rough on wireless keyboards, but with just a couple or three points in Resources the cost of new keyboards (at $25 a pop) isn't going to be an issue and when it burns out: he just waits a few seconds for the monitor to go off, grabs a new keyboard from the stack just outside the circle, turns the circle back on, turns the new keyboard on and goes back to work. I could also see a rich wizard with an entire occult library on computer a couple of floors up from his office, complete with staff; he uses an old style speaking tube to communicate clearly with them on what he wants done or looked up.
