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No problem at all, I have an inch upper on my metric lower. I did have to change the rear sight I had though to one with an apropriate height to compensate though. Best source for free info on absolutely everything FAL is http://www .falfiles.com/forums/index.php?s=
Here's a thread on your topic: http://www .falfiles.com/forums/showthread.php?s=&threadid=16099&highlight=metric+and+lower+and+inch+and+upper