possible, but......
The hitscan weapons (enforcer, sniper, etc) don't use physical objects (already said somewhere above here).
Instead, they trace a line from the start point (the barrel of your gun) at a certain angle (where you were looking at) and the trace stops when something is hit (wall, player, etc). This is where the "bullet" hits.
Because portals, in a way, stop this trace, it also stops the "bullets". I think you can change this, but it would involve some bad-ass scripting, which I doubt you want to do.