Hello, it is actually a lot more involved than that, if you want the straight scoop on it you should try the Falfiles.com. They have all the info you'll need. Most of the FALs out there are metric, so much of the info is about putting inch parts onto your metric gun. You can put inch selectors and mag releases into your metric gun, all of the threads on either one are the same. The "inch" designation versus the "metric" designation has nothing to do with the thread pitches or nuts and screws involved. That being said a metric barrel will screw right into an inch receiver and vice versa. The UPPER receiver is the registered part in the U.S., not the lower like an AR. You can easliy swap entire assemblies for the most part, uppers and lowers. Inch uppers however generally have a higher sight line and the metric rear sight won't be compatable. Some Israeli rear sights and Argentine front sights are tall like the Commonwealth (inch) guns though. Pistol grips between the two are not compatable, one slips onto a stud and the other is screwed in from the top. Although you can make the shoulder stock from one fit onto the other it is generally viewd as to big of a pain in the butt, so why go there? Handguards are more or less interchangeable. The folding charge handle off of the inch gun will work on the metric receiver with a little judicious work with the file and the parkerizing tank to make it look new again (a well worth it mod in my opinion). The charge handle can also be bought brand new for 50.00 with no mods necessary and it will count as a U.S. part too. The inch barrels use a breaching washer to time them and set the barrel depth in the receiver and the metrics don't. You can use an inch barrel and washer in a metric receiver or machine the shoulder of a metric barrel and use a washer to put it in an inch receiver. The hammer/trigger components aren't really that interchangeable, although it is amazing what a guy can do will a little determination and a dremel tool. The dust covers require a little work to interchange but it can be done. That is it for the most part, but again take a look over at the falfiles and you will be putting one together in no time.
