I have never found a way to shoot "around" RTs myself, but I use three different tactics to get at a skulk behind the RT:
1) If the RT is slightly below (i.e. you're on a slightly elevated surface), just jump up and shoot at the skulk from an upward angle. It should usually be enough.
2) Run forward, but not too close, and duck. See if you can hit the skulk's limbs from beneath. Doesn't work if the RT is lower than you.
3) Just run to the side and play ring-around-the-RT. If the skulk is slow or if you're quick enough, you should be able to hit him on the side.