As I said, you don't know what a hitbox is.

A hitbox is essentially a low poly mesh that is used to efficiently simulate physical interactions, it's used instead of the actual model because the actual models can have several orders of magnitude more faces than required to do basic physics calculations. Hitboxes can be classified and grouped to interact with eachother or ignore eachother, this is usually done for even more efficiency. Example: You could program an ejected shell casing to only interact with the floor and walls, but ignore props and player models, this could allow you to have hundreds of shell casings flying around in a complex room with almost no computing overhead. Another example: Currently, the hitbox for the barricade pieces still locked to the window frame and the hitbox for the broken pieces of barricade that are free to move on their own interact with eachother, it's probable that they are all the same class of hitbox and interact with eachother so that they look better when they pile up on the ground. They could fix the issue a couple of ways, /u/meddloiderMettigel has a solution that would need to be rephrased, but is the most simple solution: Modify the hitbox of the barricade pieces so that they no longer interact with terrain and prop hitboxes. This would mean that they would fall out of the world when released from the lock to the rest of the window/door frame though.

inb4 "HURRR NO. HITBOX MEANS BULLETS U DUMMY"