Different upper options that could be quickly swapped out like changing the upper on an AR15. Different calibers like 17hmr, 22mag. That way you wouldnt have to re-adjust the sight when switching calibers. The optic would stay integral with the upper.
Another caliber that would be perfect for the PS90 is the Tokerov 7.62x25. This cartridge has excellent penetration and can easily defeat lighter ballistic vests (class I, IIA and II). http://en.wikipedia.org/wiki/7.62x25_Tokarev
Another caliber that would be perfect for the PS90 is the Tokerov 7.62x25. This cartridge has excellent penetration and can easily defeat lighter ballistic vests (class I, IIA and II). http://en.wikipedia.org/wiki/7.62x25_Tokarev