We would need to know the model number and other specifications to possibly answer your question. As with any firearm, it is not possible to accurately appraise a gun without seeing it. Recommend that you see a local gun dealer for an appraisal. You can also do your own investigation by looking at other models being sold that are in the same condition as yours. Try the gunbroker.com or a periodical called "The Gun List" You can also reference the Blue Book of Gun Values which can be purchased on line at various retailers.
50-350 usd
Depends on what era. Currently, they used AK variants such as the Type 56, AIM, MISR, etc., M16 variants, the FN FAL, the FN FNC, and the G36.
FN
FN still makes weapons is Belgium
FN was one.
FN Herstal from belgium
FN made them as did many other makers in Belgium
$300-$450. This rifle was made either by Heym or by FN. If a true Mauser action it will be made in Belgium by FN. Otherwise it will be a Mauser 3000 made by Heym.
50-400 USD, no.
There is no firearm by that nomenclature. You may be thinking of the Swedish AK4 (FN FAL rifle, manufactured under licence) or AK5 (slightly modified FN FNC, manufactured under licence), or possibly the Norwegian AG-3 (Heckler & Koch G3, manufactured under licence).
You must call Browning. The sn doesn't follow normal convention.
Yes, it was created in Belgium by FN-Herstal, a firearms corporation, for SOCOM (United States Spec Ops Command).