I dunno if I am a gun expert but as a reformed SF weapons man, I can answer your FAL question. The weapons built to the original FN design were "metric" FALs in that the drawings they were made from were dimensioned in the metric system. They used standard metric threads and fasteners.
The weapons made in England and in some Commonwealth countries were made from drawings dimensioned in the Imperial system of inches and decimals. They used standard imperial threads and fasteners.
Parts interchange between the two is problematic. Barrels do interchange (the threads are functionally identical). Magazine interchange is one way because of the way the mags and teh slot that receives them is designed. Inch guns have a big slot and a large machined piece on the nose of the mag. Metric guns have a little machined notch in the receiver and the mag has just a little stamped dimple to it. Interchange is one way - metric mags "may" fit and feed in an inch FAL, but inch mags won't fit in the little notch in a metric gun. (a competent smith can rebuild mags from one type to the other, if the supply of surplus ever runs out.
You can see it here:
http://www.centuryarms.com/receivers.htm The guns differ in small details. Inch guns have a folding charging handle; metric guns don't. The inch gun needs a washer in assembling the barrel.
As far as skills needed, there are some parts of assembling a weapon that must be done with precision. These require precision tools and speciality instruments (the most important of which are a set of headspace gages). It's not out of the range of a normally handy guy, and the tools aren't expensive. Some stuff that auto mechanics have around (especially a good press) comes in handy.
Hope this helped.
d.o.l.
Criminal Number 18F
The weapons built to the original FN design were "metric" FALs in that the drawings they were made from were dimensioned in the metric system. They used standard metric threads and fasteners. Here is a partial list of the "standard metric threads" used in the "metric" FAL:
Bbl to receiver: 1X16 tpi (except Israeli ;>)
Screw for mag release and BHO: #10X32
Pistol grip stud/nut: 1/4X28 tpi
Buttstock double threaded nut: female 1/4X28 tpi; male 9/16X24 tpi
Handguard screw: #10X32
Front sling swivel screw: #8X40
Rear sight windage screws: #8X40
Pivot bolt: 1/4X32 tpi
Front sight pin: #12X28
Gas tube/gas block: 17/32X36 tpi
Rear support cap: 9/16X24 tpi
Belgium Long flash suppressor screw: #8X40
Israeli FALO coupler and muzzle: 11/16X24 tpi
Again, these are for the "metric" FAL: "inch" pattern rifles may have different threads altogether (L1A1 front sight, for example). For all I know, these may indeed be standard "metric" - but I have been advised (by someone who has apparently seen the dimensioned drawings ;>) that some of these 'convert' to nonstandard "metric" threads, and are so marked on the drawings...
;>)