Shooting through models

Not sure this is really a dev question, but unsure where the problem lies:

In this model pack: https://garrysmods.org/download/7390/rnl-foliage-pack

You can shoot through the trees? Is there a way to stop this - if you run fast enough you can also run through them.

They probably have anot improper physics model. You can either recompile them to have a collision box, or use the PhysicsInit* functions to create your own in Lua (they are documented on the wiki).