something I came across on the FAL files regarding inch vs metric:
"Look for the cut out at the front wall of the mag well, the cut out accepts the lug of the mag. If the cut out is a large horizontal rectangle, the receiver is cut for inch mags. If the cut out is a half moon, the receiver is cut for metric mags. Metric mags will fit into an inch pattern receiver, but with some slop. Also, look at the slot for the charging handle. If the front portion of the slot is milled away, in other words, if the slot does not extend all the way to the front of the receiver, the receiver is machined to an inch pattern. Both metric and inch pattern charging handles will install onto an inch pattern receiver. The other small feature to look for is the small cut out on the back of the slots for the top cover. Inch top covers have small tabs at the rear of the cover that will fit into the receiver cut outs. Both metric and inch top covers will fit onto an inch pattern receiver.
Timing washers on the barrel are only used on inch pattern barrels. If a metric pattern barrel is installed into an inch pattern receiver, no washers are used. The timing for the metric barrel is achieved through shoulder work on the barrel. Just remember that the thread pitch for an inch pattern or metric pattern barrel is the same."
