It comes down to the flow of actions. Pc1 hits, notices not all damage going through. Fine with him next round shifting to something different (piercing, silvered, whatever). But for his compatriot who has yet to hit this foe to know this particular creature has DR, and to shift from his standard weapon to something different to attempt to overcome the DR...that's metagaming - applying out of game knowledge to a characters actions.
Switiching from fire spell to sonic spell after witnessing your fire spell absorbed is perfectly reasonable action in the next round. Likewise Pc1 switching to his piercing weapon next turn is fine*. Pc2 already trying this out a fraction of a moment after Pc1 has discovered some DR is the metagaming I was refering to; particularly as it stems from the description of a blow another Pc made. How does Pc2 recognise the difference between DR3 and Pc1 rolling a 1 on his d6? Apparently its automatic.
* this does illustrate an in-built expectance of a level of metagaming. A novice Pc is unlikely to have come across gaming-world DR personally, whilst his player knows all about it and can have his Pc act accordingly when he thinks he's encountering it. Really when a novice adventurer strikes a foe to see the wound close up or the iron hard skin show no blemish he should think about a sharp exit, not "its only DR, I'll try silver next, Pc2 can try piercing, etc".....Most people find it acceptable that various party members will switch to various weapons once they recognise a creature might have DR, no matter the Pc's experience. Not saying there's a problem with this, just that it is an accepted level of metagaming.
Please note the distinctions between Pc and player.