LP2 Compact, James?
I'm pretty sure Steyr do a rail to fit their pistols. Probably be a 20mm rail, mind.
edit: Steyr rail: steyr-weaver-rail-short.jpg
edit: LP2 Compact, 780g plus the rail & sight: http://www.steyr-sport.com/en/steyr-lp-2-compact
edit: Ah, that Steyr rail seems to be for the LP-S, so would be best to check for further info about a rail to fit the LP2.