During my last run, it became apparent that having a version of physical barrier that can form a "tunnel" would be very helpful for some things. By tunnel, I mean a sort of "horse shoe" shape, extending from the caster to the target (limited by the force of the spell/caster's magic attribute).

My question is: Is this more powerful than the vanilla physical barrier? It does something the normal one can't, but it also can't do what the normal spell can. If different enough from phys. barrier, what should the drain code be?

If the overall effect is the same, only the shape is different, it should probably have the same Drain Code. It just requires its own spell formula, and thus costs extra Karma or Spell Points to learn if you want to know both it and Physical Barrier.

If you want to create an "Improved Physical Barrer" spell that gives you more control over the shape, I'd simply apply a variant of the Extended Area drain modifier which cranks the Drain Code up by +1 level.

Before answering, I went and looked at the wording in the book (SR3.198). The Barrier spell description allows Dome (half-sphere placed on ground) or Wall (height=force, length=magic attr).

The dome description specifically prohibits placing the "center" of the sphere-shape below ground (which would have allowed it to cover a smaller area or lower height). It doesn't say anything about casting it in a cave (earth overhead as well as underfoot) or at the edge of a cliff (does it expand to 3/4 of a sphere?) or at the tip of an antenna (does it expand into almost a complete sphere?). So the GM clearly has to make some decisions on his own if the ground isn't level or flat.

In our team, we also allow a cylinder standing on end and open at the top (radius=magic, height=force), or the wall-shaped to be placed slanted or parallel to the ground. We don't let it get used like a flying carpet, however.

Your GM might rule that the Magician can develop some additional shapes for the standard spell, or might rule that a new spell with the same drain code could be invented with 3 shapes instead of the standard 2.

Or he might allow additional simple shapes (cone, pyramid, cube, etc.), perhaps one additional shape for each Initiate grade.

Your tunnel idea is simply the wall-shape bent overhead to touch the ground on the other side, and open at both ends. If the bending was to shape a half-cylinder, it would be Force/pi meters in height, and Magic meters long. That's a pretty simple shape.