The ATF doesn't care. This is not a federal crime. They'll refer you to your local PD. And it's not "petty theft". It's burglary. It's a felony (or perhaps multiple felonies depending on circumstances). The patrol officer that takes the report is not going to be the officer that follows up on the investigation. It will be followed up by the detective division, most likely by detectives that do nothing but burglaries. Once they see the report they'll be able to compare it to other, similar incidents. This might be the missing link case they need to solve an entire series of crimes. You never know.

Maybe, just maybe, let's try giving this member some advice based on real experience so he does the right thing instead of relying on "enough stories" of dubious value. I know, I know...that's just crazy talk.

It may not be the patrol officer that enters information. As a matter of fact, it probably won't be. But it will get entered. Because if it's not there will be someone answering for that oversight once the case is picked up by detectives. I've worked at agencies where things like serial numbers were entered by various means...from officers with access to CCIC doing it themselves, to calling dispatch with the appropriate information to clerks that are assigned to do that sort of thing. Just depends on the agency. It's quite likely in this more modern age (as opposed to my dated experience) the officer might even be able to enter the numbers/descriptions from his car MDT. It's not rocket surgery.

OP...do yourself a favor. Meet the officers, give them the info you have, and let them begin their initial investigation and submit their report(s). Don't worry about what type of gun you have...not only will they likely not care what type gun you have, but they probably won't even know anything about it other than what you tell them. Most cops are not really gun savvy.