If Vincent's rifle in that scene _was_ a Belgian FNC, what sort of ammo would it be firing?
5.56x45mm/.223. Probably 55 grain Full metal jackets (easy to get and cheap) or 62 grain SS109s. One of Vincent's partners was killed by Chris in the first minute or so of the shootout, and he had a (body armor) vest on, but the threat level of said vest is unknown, so even a 55 grain could have pierced it if it was a lower classification.
I was very impressed by the looks and the sound of that rifle, and I've kept my eye out for one in the gun shops around here.
An FNC will be hard to find. What few were imported (there never was high demand for them until 1988 with the import ban looming) are probably held onto for investment purposes, and they have high prices. Good luck in your search.
Would there be any chance that a Los Angeles cop would have access to such a weapon?
Like Rex Feral said, no way that Robbery/Homicide detectives would have something as unique as a Colt Commander/General Officer handgun, or a FN FNC. SWAT would have the more unique weapons like single action pistols (I thought that the LAPD SWAT had M1911 style pistols in .45) or short barrelled rifles. I had heard that after the LA shootout the DoD sent the LAPD (under a program offered nationally to police departments for the cost of shipping) several hundred M16A1s (like 800+). If I remember correctly, they will be in the trunks of patrol or shift supervisors, and not every patrol car. I imagine that their SWAT team received some too. However, this news isn't so groundbreaking, since many of the M16A1s are going to be replacing older ones that the LAPD purchased long ago. I had also spotted a Ruger AC556K (short barrelled Mini 14 in selective fire model) in the hands of an LAPD SWAT member, but I don't know How long ago the pic was taken and if they still issue it.