Every single time (a lot of times) that I've taken NFA stuff to the range, someone has asked to see my paperwork. Once it was a local sheriff, the rest of the times it was a range officer.
While I accept that it's not *required* that I show it to any of them since they aren't ATF, why not just humor them so they'll go away? For the RO's they could say, "that's fine, you don't have to show me your paperwork- but you also don't have to shoot here. Have a nice drive home."
Do you usually not carry your paperwork? What if, however unlikely, it was an ATF agent that showed up asking?
With that being said- "something to charge this jackass with" is totally unsat on his part. I'm interested like BillM if you're going to up-channel that one. Hopefully you can get him educated on the rules, or at the very least have someone teach the guy some manners.