Philipo,
From the measurements shown on your digital calipers I calculate that the thread is imperial and is 5/8 x 24 UNEF which according to Machinery's Screw Thread Book is 0.625" x 24 TPI x 60 degree thread form, Unified National Extra Fine.
Used as a thread for suppressors and muzzle brakes on barrels with muzzle diameters of 18-22mm.
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