D&D 5E Ready action before combat starts? Advantage on initiative?

[Celtavian]

I disagree. Readied actions are resolved right after the completion of the triggering event. If both sides have chosen the same trigger, i.e. the opening of the door, then the resulting fire would happen simultaneously, with both sides possibly taking casualties. If players are prone to readying such actions, however, I would tend to set up situations where they end up firing upon friendly creatures.

I'd probably do it this way too because it seems natural and fun. Two sides unloading at each other as soon as the door opens. Then again players would probably crack the door, use it as cover and drop a fireball inside. Bastards.

 

[Li Shenron]

Any thoughts on this?

Just using the surprise rules is adequate enough.

When one side is aware it is also ready. There is no need to complicate the game by making a distinction between being aware but not ready VS aware and ready.

 

[Ristamar]

I also think using the basic rules for determining surprise are quick, easy, and applicable in this situation.

I'd have the bandits make a Stealth check, possibly with advantage. Use the lowest roll, or just roll once if they all have the same scores, and compare it to the PC's passive Perception scores. If any of the PC's beat the opposed check and the party is hesitating before anyone opens the door, they will have time to warn those that failed the check. Alternatively, if you have at least one player who is actively pushing onward without a lot of group discourse before every move, only the PC(s) that won the opposed check will avoid being surprised when the door is opened.

If you're looking to force the drama ("Wait, don't open that-" *cue hail of arrows as the door swings open*), save the opposed check until the exact moment the door is opened.