Re: Two stage triggers
I have had two of the Armalite 2-stage "Tactical" triggers and they weren't all that great. Heavy first stage of 4lbs then an almost 3lb pull on the second stage. Way to heavy on both IMO! Don't judge all 2-stage triggers by the Armalite standard! Some of their triggers are excellent out of the box while others leave a lot to be desired.
In terms of the operation of a 2-stage trigger...obviously, it has two portions that can be felt during trigger pull.
The first stage has a much longer, generally heavier travel than the second stage. The first stage is normally called take-up (and some other synonyms) but should not be confused with creep or other slop in the trigger. At the end of the first stage, you'll experience a definite stop. This is the beginning of the second stage. At this point, you have "staged" the trigger and are ready to engage the second stage and fire. The trigger essentially works like a single stage trigger at this point. You should have a crisp, clean break with around 2.0lbs (give or take) of pressure.
The advantages are primarily one of safety and then one of accuracy. An AR running a single-stage trigger will get you into a trigger like that found commonly on bolt action rifles. No creep, no slop, minimal reset, just apply pressure to the trigger and then break like a glass rod with around a 3-3.5lb pull or so. The 2-stage on the other hand offers safety and accuracy. You get a long, first stage with around 2.5lbs pull then a second stage with a crisp, clean pull similar to a single stage trigger, but with only a 2lb pull (give or take). You can't safely get an AR single-stage trigger much below the 3-3.5lb pull mark because of the risk of AD's, bump fires, etc., etc. The 2-stage triggers (some) are infinietly more adjustable to get a truly "tuned" trigger setup to the individual shooter's preferences. Take the Giessele for example...you can get a "Match" 2-stage trigger with a 2lb first stage with a 12oz second stage (infinitely lighter than anything you can get close to with a single-stage trigger). I run Geissele DMR triggers where I get a 2lb first stage and a 1.5lbs second stage...it is the cat's meow for AR triggers.
If you are used to a quality, tuned bolt action rifle trigger (single stage), then getting used to a 2-stage trigger can be a real stuggle and can take a while to get used to.