FAL's fall into two basic categories: "metric" and "inch pattern".
Metric FAL's for the most part are built to specifications set forth by FN when they originally designed the weapon. Various parts may have been updated/revised over time, but the basic dimensions and tolerances were established by FN using the metric system.
In the early 1950's, when the British Commonwealth decided to select the FAL as their next generation rifle, a "Rifle Steering Committee" was established with representatives from Britain, Canada, and Australia. The RSC took FN's design, and converted the drawings from metric to inch dimensions. They revised a number of dimensions minorly, and they altered some of the tolerances on the basic dimensions as well. They also changed most of the threads on the various parts to imperial thread from metric thread. The end result was a rifle that was a "close copy" of a FAL as made by FN or it's liscensee's, but not an EXACT copy.
Some parts WILL interchange between metric and inch pattern FAL's, but there are a number of parts that will NOT, especially anything with threads involved.
For more details on this, see R. Blake Stevens series on the FAL, available from Collector Grade Publications.
