Thanks for the clarification.
It isn't exactly a great perk. It would be a downgrade of a job for former police as it wouldn't pay as well as a police officer with a long length of service. Given it doesn't pay that great (if you are hired as a civilian rather than police doing it) and it is hard to find anyone to do any job right now, it's unrealistic to expect FEOs to have a highly detailed knowledge of every caliber to the level that someone interested in firearms and for who firearms are their primary hobby or profession, would have. Former forces and former police should have some familiarity with firearms, law and policing, they seem a good choice for FEOs. Although I have had 2 FEOs and both have been serving PCs.
Some lack of highly in depth knowledge may be just as well or the police would start to ponder why you can have a 300 PRC or 300 Norma which will have about a 50% hit probability on a man size target at a mile, vs a notionally more powerful large bore hunting caliber which won't be hitting anything at half that distance.