Some background first…
We had a discussion in our group about object HP and using that as a kind of temporary hit points. It came about when my halfling was hiding in a barrel and a bandit hacked the barrel to pieces to get to me. In the next session another player showed up with a length of chain fitted to his armor. Our DM saw he'd described it well enough to show he'd given it serious thought and let him have the 10 HP of the chain.
The DM used this to his advantage by having the first strike of the first encounter hit that character and applied the damage (which was a secret roll that happened to deal 11 damage), that broke the chain. This worked to everyone's satisfaction: the player got to feel like his clever idea did something (saved him from a powerful attack), the DM got to make his encounter seem more threatening and he restored the game to the pre-chain state. So far, so good right?
A third player in the group decided he wanted to set up a chain like the other character, the DM ruled he couldn't because (a) they didn't have the time (the other chain was placed between adventures) and (b) the character didn't have the skills/tools (the one with the chain has smiths tools or something- I can't remember, they aren't my characters). Problem avoided?
The next session the player who didn't get to use chains came back- he'd picked up the tool set between sessions and basically maxed his encumbrance on chains. First encounter- the DM strikes him, 11 points of damage ALL of the chains he's wearing are cut free… the gold spent on them is wasted. He rules- no matter how many chains were attached to armor, the character can only benefit from one.
The DM allowed the 1st chain because the player was inventive and thoughtful, so he ruled by fiat. I think if we had all tried the single chain he would have allowed it, but this (wearing a dozen or more) was obvious abuse.
So now I've been thinking… I can't find an "official" rule for or against doing this, the closest would be the cover rules, but I think in this case using the object HP ends up being less of a mess than trying to use it as transportable cover (which would give an AC bonus etc). Does anybody know of an actual rule that would cover this?
Best Answer
It's called Armor.
What do the Rules say? I think the applicable rules are armor rules. The intent behind wrapping chains around yourselves is protect yourselves. That is the job of armor.
Chain fashioned around your self are similar (enough) to Chain mail or ring mail, both are heavy armor. Chain mail requires strength of 13 and imposing disadvantage to stealth. For that you get an AC of 16. Those chains have no HP, and no mechanism for it to be destroyed for simplification reasons.
Adding other objects to the character to act as armor isn't necessarily outside the rules, but isn't something the rules consider. Which means that what happens is left up to the DM. The most appropriate rules for this sort of thing are the armor rules, not the object HP rules. Adding armor in this fashion should affect AC, not add additional HP, and depending on the amount of chain likely have stealth implications.
Object Rules
The barrel is what item HP are meant for, a situations that happens to come up where someone needs to destroy an object. The rules on objects are for objects, not armor. The rules for objects can be found in the SRD ( official ) in Game Mechanics under Objects. Reading them it is clear that the intent is an item caught in the cross fire, or the characters try to destroy an object. It isn't meant for items that are worn to provide temporary hit points.
This path might inspire your DM to employ armor/weapon damage house rules. Your swords might suddenly break or shields shatter. That is the logical extension of applying these HP rules to all things in the world.
Ultimately, it's the DM's Call
To apply armor rules or object rules is the DMs choice. Armor rules are the ones intended for defense, but there is nothing preventing him from applying object rules the way he has already.