True. In that case, they could maybe use this guide: How to Make a Barrier that Doesn’t Block Bullets [1/10 or 🟦]
which is basically this:
This should allow the sentry to attack the player while the barrier is still active and has collision. However, I think the collision is mainly intended for players only, because I have never tested if sentries can stand on it and attack through it.
In that case, if they wanted a barrier that blocks player’s bullets but not sentries’ bullets, they could use this guide: How to Make a Barrier that Doesn’t Block Bullets [Difficulty 1-2/10 or 🟩]
This is because: