That is not really the case mate, the metric rifles (the most common) are all pretty much interchangeable as are all the inch rifles (the latter was a REQUIREMENT of production in Aus/Britain/Canada - why they are also known as ABC rifles). There is also some interchangeability between metric/ABC rifles. There are some exceptions of course and I don't deny that but pretty much all the bits between a Belgian/Austrian/Brazilian/South African/DSA rifle will interchange. The Argentinian and Israeli rifles can throw a couple of issues into the mix but they also are mostly interchangeable with the rest of the metric breed. Don't even ask about the Indian rifles.