vehicle vs. home makes all the difference
http://en.wikipedia.org/wiki/Carroll_v._United_States" onclick="window.open(this.href);return false;
As for "fishing" - again, it's a grey area, fine line, whatever euphemism you like.
Police can ASK you any question they want. You may or may not be required to answer, depending on circumstances. You may feel pressured to answer, but as long as the officer is not using undo influence to force you to answer something you're not required to answer, what has the officer done wrong by asking? Honestly, I've seen car salesman much more high-pressured than what I gather from the description of this incident.
A 10-minute detention is likely within customary time for a traffic stop - and how do any of you know that checking the gun was the ONLY thing the trooper was doing during those 10 minutes? Maybe he was also checking OP's DL, license plate, etc (which is also customary - after all, this is how they caught McVeigh as just one high-profile example).
I know none of us LIKE the idea of the government checking out firearms serial numbers. And I've not one time defended that action (merely stated repeatedly that there COULD have been a legit reason, we just don't know) - but I agree it's unorthodox. The rest of the stop, however, seemed perfectly reasonable and customary and for his cooperation the OP was given no citation for TWO admitted violations of law - speeding and no front license plate.
The key point to remember in all this is that because the roads (most of them anyway) are public utilities and because you MUST be licensed to drive on public roads, you do give up SOME small measure of total leave-me-alone privacy when you venture onto a public road. A car IS NOT the same as a home.
I really think this whole thing boils down to checking the gun serial number and no one would be complaining about any of the rest of these red herrings if not for that one wrinkle.