ConcreteBuddha said:
True, this is one of the ways to reach absurdity. Another is to make up a rule that directly contradicts the rules as presented.
So stop doing it.
You have just defined the word "hardness" differently than they do in the PHB.
The PHB is not the final arbiter on the use of the English language, even in a D&D campaign.
Also, you have used real-world science on a make-believe substance. Shame on you.
This would be relevant if I was using it to undermine the rules. Since I'm not, you are simply flailing around.
I look forward to it. You seem to be channeling Magus_Jerel quite well. Perhaps you should now insert a reference to quantum mechanics and start Capitalising References to Yourself.
That's because your interpretation leads you to that conclusion. My interpretation, on the other hand, does not have that inconsistency because adamantine arrowheads do not give an enhancement bonus.
They do have the rather silly corollary that making the peripheral bits out of adamantine has an impact on a weapon's effectiveness, though.
Which shows they have different base damages.
And you don't find it a tad absurd that both an arrowhead and a greatsword receive the same enhancement bonus even though a greatsword weighs at least 300 times more? (15 = .05 x 300)
First, as I said (and you conveniently snipped) the weight of the greatsword is completely out of whack. And second, no; any more than a longsword and a greatsword both gain the same bonus. The exact mechanism by which an enhancement bonus works is left undefined in D&D; what really matters is the complete package, and that means looking at the base damage as well.
Even when the description states, in full:
"...this ultrahard metal adds to the quality of a weapon or suit of armor based on how much of the material is used. Thus, adamantine plate offers a greater increase in protection than adamantine chainmail, and an adamantine battleaxe offers a greater increase in offensive capability than an adamantine dagger." pg. 242 DMG
And if you call this passage "flavor description" and therefore utterly meaningless
Strawman. I didn't say it was "utterly meaningless". I said that as a general description of the intent of the rules, it was possible that specific situations could contradict it. That's the nature of generalisations.
(as per the spearhead thingy...)
Well, if I take a steel spearhead and interchange it between a halfspear and a shortspear, I only change the base damage.
But when I take an adamantine spearhead and interchange it, the enhancement bonus changes, too.
So don't allow it. Or whack on a massive Craft DC check to stop people doing it, if it bothers you so much.
People can do this with steel spearheads too. Do you have a problem with that?
And I repeat my previous stance: since there are materials less efficient as a haft, it seems plausable that there might be materials more efficient as a haft.
Such as?
Let's look at a spear:
An adamantine head and nerf foam shaft would be really inaccurate because it's so flexible, and it wouldn't be able to damage very well because it would bend under the weight of the attack.
A adamantine head and a 10 foot long wood shaft is still somewhat flexible, and therefore it is difficult to aim with pinpoint accuracy, and you would damage an area less vital.
And adamantine head and a 10 foot long adamantine shaft is not flexible at all. You can easily aim attacks at vital regions because the shaft does not bend.
This is a silly argument. A 10-foot-pole as used in a pike is not "flexible"; these things aren't made of balsa wood. It may have the ability to bend to avoid breaking, but it isn't going to flop around either. That goes double for a 3-foot-long battleaxe haft, or an arrowshaft.
Explain to me why anything other than the very tip of a short sword would have to be adamantine to gain the enhancement bonus with your interpretation, considering it is a piercing weapon.
For the exact same reason that regular shortswords aren't made of wood with a metal tip. Can you find anything to suggest that regular shortswords aren't made that way?
The rules leave some things undescribed because the designers assume _some_ level of familiarity with how the real world works. There's nothing in the books that explicitly says characters have to breathe or drink or pee either, and yet we blithely assume they have to do all these things.
Well, don't you think if the game designers meant for weapons to be partly fashioned from adamantine, they would have said so?
If the game designers meant for weapons to be fully fashioned of adamantine, they would have said so. They did for mithral and darkwood, but not adamantine. They didn't have to, because it should be self-evident that not all portions of a weapon contribute equally to its effectiveness.
Seems like a pretty big exclusion from the rules, considering that using your interpretation, you can make multiple different, yet effective, versions of the same weapon using extremely varying amounts of adamantine, and get the exact same price for each version.
And by the rules, you can have use-activated items of true strike too. The rules are not perfect, especially when it comes to creating magic items.
Actually, my stance agrees with both the amount of material guidelines and the damage chart. Yours only agrees with the damage chart. So who's interpretation is more correct, since we are using circumstancial evidence?
You might as well ask why a use-activated item of true strike shouldn't cost only 2,000 gp, since that's what it says in the rules.
Since this is as close as I'll get to a retraction, I'll take it.
You can take it any way you want, if it makes you feel better.
Sure, you could make fully adamantine arrows or arrowheads. But adamantine arrows could not effectively fly using normal bows(since they weigh the same as steel arrows)
Chapter and verse, please. You're the one who seems bent on a ridiculously narrow position when it comes to interpreting the rules. I see nothing in the book that requires that adamantine arrows couldn't fly.
Actually, no you didn't. Because:
A) Leather armor can be made with adamantine buckles.
B) Leather armor is light armor.
Therefore, by your rules, leather armor with adamantine buckles should gain an enhancement bonus, regardless of how many buckles it has.
This is a doubly silly example. The buckles are not the significant portion of the armour _when it comes to protection_, so making them out of adamantine would have no impact on the armour's ability to protect. It's just the same as making a spear shaft, but not the head, out of adamantine. As I've said twice already, your example is irrelevant.
Actually, I made a mistake. A person could not make adamantine leather in the first place because it would break:
3) Items that cannot be fully crafted from adamantine and still remain effective cannot gain the enhancement bonus.
Where on earth did you pull that quote from? It's not in the DMG description of adamantine.
True, the DM has free reign to assign how much damage an iron arrowhead would take, but does not have the freedom to assign hardness and hit points. These are already a part of the system.
So? Your point was that the "rules" prevent adamantine arrowheads breaking. Since this would require rules for damage taken as well as durability, your point is wrong.
Well, the rules are the be-all and end-all in this forum. Hence why "it's magic" is worthless in this forum. Unless you'd like to move this to house rules...? 
I'm not saying that "it's magic" is a substitute for the rules. I'm not even saying that "it's magic" is why the rules are wrong. I'm saying that a plausible in-game rationale for why the rules are as they are, is a good thing. Assuming that the people playing the game aren't robotic automatons who have no concept of an in-game reality beyond the rules, that is.
Notice how the PHB's version of hardness applies to all situations, including driving cracks though it or projecting it from a bow. Also notice, that in DnD, diamonds would have a really high hardness and really low hit points. Whereas adamantine has both high hardness and high hit points.
Sheesh. If reusing adamantine arrows really bothers you that much, just slap a massive Craft DC check to affix the head to a new shaft. All the "hardness" thing was, was a handwave to explain to recalcitrant players why these arrows might not be reused. As with such things, multiple handwaves are always possible. Pick one to fit your tastes.
Interpretations of rules are house rules?
If a rule is ambiguous enough to lend itself to multiple contradictory interpretations, then choosing which interpretation to apply is something that depends on the individual DM's tastes, and hence is by definition a house rule.
Amount of adamantine in an arrowhead leads to a glaring contradiction.
Hardly.
Shattering adamantine combined with DnD hardness definition leads to a glaring contradiction.
Not at all, and even if it did, you could use something else to solve that particular problem.
Allowing ammunition to be made out of adamantine, even though the rules state that ammunition does "---" damage.
You ARE channeling Magus_Jerel quite well.