The 6-8 lb AR trigger was designed the way it was or a number of reasons. It is simple, reliable, interchangeable in mass production and most importantly from a military point of view - safe. It will not fire on safe, and it will not fire when dropped. Pretty much any milspec AR trigger component will work safely with any other.
The condition of the searing surfaces is critical for safety and trigger feel. There are very tiny grooves machined into the surface when ground that to an experienced shooter feel like speed bumps. This surface irregularity and the length of engagement is known a creep. This surface irregularity holds oil for corrosion protection and provides friction for drop safety. The trigger provides the leverage to overcome the friction.
These surfaces can be polished - at the risk of promoting corrosion and making the trigger unsafe if dropped or jarred. Idiots often polish these with Dremels or stones right through the case hardening which allows erosion and an ever decreasing margin of safety.
Some bench or match shooters are not worried about these things, but service shooters should be.
Match triggers evolved to improve the feel which can be subjective in the absence of something like a Dvorak machine.
http://www.dvorakinstruments.com/Products/TriggerScan-System.asp This charts pull and feel over the entire length of the pull and records release points and multiple stages etc.
They also improve consistency. This can be measured easily on one of these with 5 or six pulls:
http://www.lymanproducts.com/lyman/tools/trigger-pull-gauge.php Nearly any match trigger will have a much smaller range of trigger pull weights
Keith shoots with a match trigger - for a reason. Anyone that knows how to shoot can understand that trigger squeeze and follow through are important. Set triggers one single stage and two-stage triggers evolved to lighten the point of release and make the trigger more consistent. If the trigger is smooth and the same every time you are free to concentrate on sight picture. As mentioned before this hides shooter inconsistency. A poor shooter will see much less benefit than an experienced shooter.
They all cost more money that the standard trigger, but don't necessarily make improved groups or scores.
Adjustable: Some match triggers are designed for adjustability so the user can tune a number of preferences such as how much slack is in the trigger, the force required to release the shot and how much over travel is in the trigger after release. These are often complicated and anything that can be adjusted will eventually adjust itself or be adjusted by an unqualified individual. Also many adjustable triggers can be adjusted to go off by themselves or not at all. The
KAC - one of the best feeling triggers by my estimate has an 8 page installation manual.
Single Stage: Usually involves a conventional searing surface that has been shortened and or/polished. The ALG is a standard AR trigger with a nice coating and a polished surface. A
Timney trigger has very little movement and nearly no over travel. This means re-set and follow up requires minimal finger movement and less movement overall.
Two Stage: A two stage trigger will have a first pull, in the case of the
Geisselle SSA will be 3.5 lbs and a "stop" that you can feel. The second pull will release usually after a smaller and more polished searing surface disengages. In the case of the Geisselle SSA this will be another 1 lb, and have a relatively short movement and a relatively limited over travel.
Many two stage match triggers dispense with the standard AR searing surface location and use one or two spring loaded disconnects on top to release the hammer in stages. Like the
RRA. Even the simplest one will provide a consistent and lighter release, however this is often at the expense of safety as spring loaded sears can move under drop loading and fire. In the case of the Geisselle SSA, one disconnect is spring loaded, the other is fixed portion of the trigger (like the RRA) and a standard AR searing notch at the bottom is retained as a safety notch. This one is drop safe, many match triggers are not safe for some uses.
This trigger is in use operationally - including by police and military units that may fire at NSCC and CAFSAM - because it is safe and consistent. Two stages and limited over travel means that to fire the first shot you can take up the trigger to the first stage (3.5 lbs) and the release will be easy (1 lb) and consistent. A follow up shot will require less forward movement of the finger to re-set and the shot will require only the second stage (1 lb).
You won't be able to measure the improvement just in group size, but there can be tangible benefits from better scores due to increased consistency, faster split times etc. Taking up eight pounds of a full movement trigger at the end of a snap exposure vs taking up one pound with limited movement could be the difference between seeing backstop when the shot releases and seeing a nice sight picture.
