Break DC too low?

Question

I am making a Magic Rod out of Adamantine. The normal description of a rod is as such:

Physical Description: Rods weigh approximately 5 pounds. They range from 2 feet to 3 feet long and are usually made of iron or some other metal. (Many, as noted in their descriptions, can function as light maces or clubs due to their sturdy construction.) These sturdy items have AC 9, 10 hit points, hardness 10, and a break DC of 27.

Now the Rod I am making is 1.5 in diameter, which yields an HP of 60, and Adamantine has a hardness of 20. I also understand that adding magic to a rod will increase the HP and Hardness of it.

Hardness and Hit Points: Each +1 of enhancement bonus adds 2 to the hardness of a suit of armor or shield, and 10 to its hit points.

With this being the case, the weapon I am proposing has 6 times the HP and twice the Hardness, how is it that it still has the same Break DC? The only rules I see on the calculation on Break DC is:

Based on PHB Pg. 167

When a character tries to break something with sudden force rather than by dealing damage, use a Strength check (rather than an attack roll and damage roll, as with the sunder special attack) to see whether he or she succeeds. The DC depends more on the construction of the item than on the material. For instance, an iron door with a weak lock can be forced open much more easily than it can be hacked down. If an item has lost half or more of its hit points, the DC to break it drops by 2.

Based on RC Pg. 107

When a creature tries to break something with sudden force rather than by dealing damage, use a Strength check (including a modifi er due to size) to see whether it succeeds. The DC depends more on the construction of the item than on the material.
Table: DCs to Break or Burst Items
Strength Check to: DC
Break down simple door 13
Break down good door 18
Break down strong door 23
Burst rope bonds 23
Bend iron bars 24
Break down barred door 25
Burst chain bonds 26
Break down iron door 28
Condition DC Adjustment*
Hold portal +5
Arcane lock +10

Based on Pathfinder: d20pfsrd.com/equipment/damaging-objects

Breaking Items
When a character tries to break or burst something with sudden force rather than by dealing damage, use a Strength check (rather than an attack roll and damage roll, as with the sunder special attack) to determine whether he succeeds. Since hardness doesn’t affect an object’s Break DC, this value depends more on the construction of the item than on the material the item is made of. Consult Table: DCs to Break or Burst Items for a list of common Break DCs.
Table: DCs to Break or Burst Items
Strength Check to: DC
Break down simple door 13
Break down good door 18
Break down strong door 23
Burst rope bonds 23
Bend iron bars 24
Break down barred door 25
Burst chain bonds 26
Break down iron door 28
Condition DC Adjustment*
Hold portal +5
Arcane lock +10

Are there any other rules then this?

Edit: I was asked to change the wording of the question from pathfinder rules to 3.5. Personally I see them saying the exact same thing with different language. For sake of having information for as many sources as possible, I am adding the PHB and the RC rules. Once again, I do not see them as stating different information, just saying it in a slightly different way.

As a follow up question, does anyone feel the answer to this question changes between the 3.5 Rules and the PF rules?

Answer 1

Break DCs generally don't reflect breaking the material, they reflect breaking the structure the thing in question is part of. Material can still be relevant, but it's not very explicit as to how in the books.

Preliminaries:

1) I say generally because it refers to bending iron bars. However this kind of applies to breaking a structure, because I'd be willing to bet that this refers to a strict kind of iron bars system such as in a jail cell, where the bars are fastened at both ends and are at least a certain bit long. This answer is more or less "there's too many factors to make a formula for break DC, use your best judgment", analogous to estimating some magic item costs (as outlined in Tome & Blood).

2) I don't know of any material for extra rules on Break DCs, but tables of Walls and Doors with their break DCs and more can be found in the DMG pages 60/61, and are discussed below. There might be more such tables.

Main Argument

First I want to emphasize that when you're looking at these DC's, it's an attribute check from the perspective of a medium creature, and it scales fast. For medium creatures, a DC 30 strength check is really high: you'd need at least 30 strength for the +10 modifier, then you have to roll a 20 to boot. A hill giant with 25 strength could be put in a set of masterwork manacles because his +7 modifier wouldn't suffice to beat the DC 28 break check. Reinforced masonry walls have break DC 45. The Tarrasque, with 45 strength, could get a max 37 strength check.

Next, the construction aspect. For example, look at the table for doors on DMG 61. The break DCs for wooden doors of different strengths don't reflect smashing through the door, they would represent hitting the door hard enough to break its hinges or something of the like. A strong wooden door is thicker and has a higher break DC than a good wooden door, but it's not because it's "more wood to break through". It would be more like because there's "more wood to hold a better hinge/lock/etc", which is why a locked iron door 2" thick only has break DC 28 compared to a locked strong wooden door at DC 25.

For walls though (DMG 60), material definitely factors in on top of the construction aspect. I doubt construction alone explains why a 6" wooden wall has a break DC 20 while a 3" iron wall has a break DC 30, with no involvement of iron vs. wood. A DC 20 wall keeps out the average joe with 11 or less strength, while a DC 30 wall keeps out frost giants (at least in terms of breakability), to further emphasize the scaling of these break DCs.

In your case, the break DC of 27 for a rod would probably be for iron rods of a "typical" thickness, which isn't specified. So when you say you're making a rod, it'd be assumed that you're making one of the typical thickness. How the material hardness changes the break DC is realistically a DM call, but 27 is already quite a tough DC, where you need at least 24 strength to break it. A hill giant would have only a 5% chance of breaking it. Does adamantine as opposed to iron mean the rod is tougher? Probably. Tougher to the point where not even frost giants can break it? Probably not. So maybe the break DC would be 1 or 2 points higher, if at all?

Guideline for Figuring out Break DCs

Establish "typical values" if needed. For your rod example, maybe a typical rod is 1" in diameter and made of iron. Then yours being 1.5" in diameter and made of adamantine may both contribute to a higher break DC. But try to be mindful of what kinds of creatures could break the typical item in question, and how much stronger items that deviate from the typical values might be, based on the quick scaling of 8 PC levels per point of DC or the scaling of general monster strength (hill-giant proof -> frost giant proof -> tarrasque proof -> etc).