I would be interested to hear from a new M4 owner if the trigger is actually any different (better) and if it it retro upgradable to the mk1, mk2 and M3. It is the only aspect of the M4 that if it actually is a better unit would interest me. My MK2 has all the M3 upgrades (apart from 2nd Reg) and is fitted with the Airmarksman rail. The only thing I'd like to try is a solid (LW?) barrel, but they currently only make them in 14mm .177.