Is there a way or a tutorial on how to convert from a standard model to FOV?

You have to manually make the end of the gun larger and longer as far as I know.

In 3dsmax you can use the taper modifier.

I've found that if you taper first from the barrel end down to the stock first, then from the stock to the barrel end, it produces the best results IMO. You also have to make the object more squashed together near the stock end, and more stretched towards the barrel end. Look at a default Model in HLMV, to get some idea, of the overall shape.