[b] They must keep C3 paperwork with the weapon. [/b]
I am issued two FA weapons by my employer, and I don't have to keep any paperwork with them. Its not anything that has every come up. We often shoot a local private ranges (ammo and fees provided by the Department) because we do not have a range ( a long story). I have been asked for the Class 3 paperwork on the weapons by range, but I always just say that they are Department weapons and I show my badge and ID; no problems. I asked an ATF agent about how to determine a legally owned Class 3 weapon and he said that while there is no requirement to do so, a Class 3 owner should keep a copy of their papers with the weapon if they are transporting it to avoid problems. Privately-owned Class 3 is way out of my price range, so if any board members have better information, I wouldn't mind hearing about it.
It does sound like this guy came off a little superior. I look at shooting on these private ranges as a good opportunity to do a little PR work; the gun owners on the ranges are generally very supportive of Law Enforcement. I hand out business cards and show off my issued "toys." I also let them know that most LE (at least in my area) are supportive of gun owners (we are all pretty major gun owners ourselves). They like the fact that we are friendly and get a lot of training. They are usually jealous of the fact that I am getting paid to shoot and have issued MGs, but hey, it is a trade off for the fact that they all probably make more money than I do...
My Department also only wants us to shoot duty ammo. This gets us some strange looks as we launch off several hundred rounds of $25 a box ammo at the range. When asked, I always respond "Our Department only wants us to shoot the expensive stuff through our expensive weapons, and they give us as much as we can shoot, those bastards."