I have the same gun. My first dedicsted hunting rifle. I had it cut down to 18 inches and threaded 9/16 x24. Then found out the standard was 5/8x24 for .308. At the time I only had a L1A1 and this in .308 and they were threaded the same, so no big deal. Eventually I sold the L1A1, and put a 9/16x24 to 5/8x24 adapter on the barrel. Then I standardized on 13/16x16 adapter, to make it easier to swap around the suppressor.
Any way, at 18 inches it did quite well for me. Killed my first three white tail deer with it. A doe, an 8 PT and a 10 PT.