National match trigger pull is 4 to 4.5 lbs, this the minimums set for national guard shooting teams, by the US military.
The armorers manual warns to not go below 4 lbs as an absolute minimum to stay within the safety parameters of the rifle design. Trigger pulls under 4 lbs can cause lock up problems during cycling.
For potential liability issues, I will not adjust a clients trigger to less than 5lbs, the recommended minimum pull weight for civillian rifles.
There is no shroud of secrecy around trigger jobs but they should either be done by a professional or under the supervision of one. Do it without a clear understanding of what you are doing can lead to bad things. As Master Hungry likes to say, your #### might fall off LOL