A true inch pattern FAL is less common on the used market. They would generally be from British commonwealth countries. The L1A1 is the most common. The major difference is the magazines. The metric magazines are far more common, and many "inch pattern" civilian L1A1's were built on imbel metric receivers so don't work with inch mags. There are other parts that are not interchangeable as well. I believe the metric mags fit the inch rifles but the inch mags wont work in most metric rifles.
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Below is from DS Arms website FAQ
https://www.dsarms.com/t-faq.aspx#divForm
- How do I tell the difference between a Metric and Inch pattern?
- L1A1 Barrel profile at the breech has a longer taper.
- On the Metric the gas block ears are closed on both. The L1A1 Gas block ears are open on each side & front sight post has a set screw at the face of the gas block.
- Magazine tooth on the L1A1 inch pattern is a much larger squared protruding lip. The metric has a small radius protrusion. The corresponding relief in upper receiver will be cut as a deep channel on the L1A1 all the way across the inside of the engagement surface. The Metric upper receiver will have a small semicircular notch in this area that will match the protrusion on the Metric magazine.
- The L1A1 Lower receiver has extensions where the buttstock meets the back face of the lower/trigger housing. These extensions have a radius as they form into the buttstock tang. On the metric the back edge is straight vertical line. Also on the L1A1 the trigger plunger assembly is housed in the pistol grip. On the metric rifle the trigger plunger is mount on protrusions on the bottom of the lower. The internals are also different.
- L1A1 Bolt hold open shaft is slightly larger than metric shaft.
- L1A1 charging handle slide bar is slightly thicker than the metric version. L1A1 can be shaved down to fit a metric receiver.
- Bolt & carrier groups will interchange as complete assemblies however extractors and firing pins do not interchange.
- Muzzle end barrel threads are 9/16 24 TPI right hand on the L1A1 but left hand on most metric versions. Some Belgian rifles can be found with either or. Early Belgian rifle can be right hand.
- Top covers on the L1A1 have 2 protruding ears on the bottom rear that fit into recesses milled into the upper receiver. Metric top covers do not have these ears.
- The above does not list all the differences but does cover the major ones. For more, please refer to the FAL Rifle Book by Blake Stevens.