I understand the 5e move to simplicity eliminating size modifiers for combat. 

That said I find it a little off when dealing with certain things. For example, making attacks, particularly ranged attacks against tiny or huge objects or ones with unusual silhouettes. I think one other issue is the AC system conflates "hitting" and "damaging." It should be relatively easy to "hit the broad side of a barn" and not harder to hit the "broad side of a stone castle."  

Ex.

1. Robin Hood is in an archery contest.  The straw target is AC 11 (I guess) which is the same AC as the whole bale of hay. 
2. The PCs place a black powder keg at the entrance to the ramshackle mine. The wizard aims his firebolt at the fuse. The keg is AC 13? which is the same as the large door next to the cave. 
3. The PCs want to throw a flask of oil on the door. It seems weird that it should be harder against a metal door than a wooden one. 

As DM I can arbitrate different numbers or make it something else some kind of check maybe? I wish there were some standard so the PCs could figure their odds without me just guestimating.

As a DM, you can arbitrate all numbers and ratings in the game, both at a larger degree and at such a small, specific scale.

I mean, as a DM, you can choose to ignore or change anything in the game, so this is a low bar.

Problem is that Damage Reduction from previous editions has partially been stripped out of certain sections of the game.  Objects made of harder materials should have Damage Reduction based on the type of material and the type of damage.  

Quote from Malzerious >>

I understand the 5e move to simplicity eliminating size modifiers for combat. 

That said I find it a little off when dealing with certain things. For example, making attacks, particularly ranged attacks against tiny or huge objects or ones with unusual silhouettes. I think one other issue is the AC system conflates "hitting" and "damaging." It should be relatively easy to "hit the broad side of a barn" and not harder to hit the "broad side of a stone castle."  

Ex.

1. Robin Hood is in an archery contest.  The straw target is AC 11 (I guess) which is the same AC as the whole bale of hay. 
2. The PCs place a black powder keg at the entrance to the ramshackle mine. The wizard aims his firebolt at the fuse. The keg is AC 13? which is the same as the large door next to the cave. 
3. The PCs want to throw a flask of oil on the door. It seems weird that it should be harder against a metal door than a wooden one. 

As DM I can arbitrate different numbers or make it something else some kind of check maybe? I wish there were some standard so the PCs could figure their odds without me just guestimating.

 

 

            To be more specific, I would say that:

              1) non-mobile straw target that is rougly Medium in dimensions would be AC 8 or 9.  Mules have an AC of 10.

               2) non-mobile powder-keg is about same as non-mobile straw target. 

               3) AC of metal or wooden door, if you are not trying to actually damage the door, would be AC 8 at the most. 

           Not that you asked, but since we're on the topic, hitting the bullseye on a target that is at least 90 feet from the shooter would be about AC 15 by my estimation.  For every 30 feet beyond that, add an AC of 1.  

1 Robin Hood can hit the target. But The outer ring is ac 14 2 nd ring ac16 3rd ring ac 18 and bulleyes ac 20. These ACs reflect accuracy. 

2 No problem

3. The AC could be 10 but then you would have to change damage thresholds and HP.

AC is occasionally an abstract. 

You can also consider the AC to be the chance of landing a hit which does real damage.

I've always worked on the premise that combat is a continual back-and-forward process, where the game mechanics determine how many attacks might actually land well enough to cause some real damage (or other effect) on the opponent.