Alright everyone, I'd like some help settling a debate that a friend and I have been having lately.
It involves Knowledge skills and what things characters would and wouldn't know.
I contend that unless a character has personal experience with a creature with DR/(special material), has a knowledge skill that pertains to such a creature, or has talked to someone who has such knowledge or experience, they would have no in character reason for buying special ammunition or weapons to bypass such DR.
He contends that any knowledge of DR and materials bypassing it is common knowledge, and as such such a character would have every reason to do so.
The specific example in question is regarding an archer who had fought a total of 1 creature with DR/Adamantine, which she did not ask about or question afterwards, but then went on to buy silver and cold iron arrows despite not having any knowledge of creatures with DR that was bypassed by those materials.
Now, I, as the DM allowed it because I knew it would start a long argument if I said anything about it, and such things are usually moot as the archer could just ask the resident knowledge expert "Why weren't my arrows doing anything to that beast?" and get a lecture on the nature of DR and what bypasses it. I just contend that if none of that is done, the character would have no reason to buy the special arrows.
My reasoning for this is that I can't find a creature common enough to meet the Goblin style DC 5+CR standard for Knowledge checks, so technically the weaknesses of every creature with DR/(special material) isn't common knowledge.
Thoughts? Opinions?