Yes, GP90 is the Swiss issue 5.56 ammo. Granted, it was the only round I found that would give me MOA accuracy. Most others were around 2, with even the best handload I could produce just a hair under 2. Again this was with the ring and dot optic which has a 1MOA dot in the center and planted in sandbags. My eyes were a lot better back then as well. Today I can barely even see that dot! I don't think you would get that accuracy with the standard ring optic.
Auggie D
Auggie D