As a former federal prosecutor, I can assure that BATF would not know what to do with a .505 Gibbs if once came up and bit it on the arse. We had enough trouble getting agents not to write reports mentioning ".9 mm" pistols and .410 "gauge" shotguns, which they then expected us to present to a Grand Jury.

I'm afraid you would not get far alleging fraud in a case where a firearm failed to feed a cartridge correctly. Since the rifle was manufactured in the Czech Republic, presumably it underwent proof testing and was certified safe to shoot with standard ammunition. That should be enough to mount a successful defense against a charge of negligence, let alone fraud, which is an offence involving criminal intent. Case dismissed!