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What is the reason for a 2 stage?
They are safer and they have been standard equipment on all US military rifles until the M16. The M16 did not have it because Eugene Stoner was lazy.
It's a different feel. Some prefer it. A two-stage rifle trigger feels like a 1911 trigger -- first you have some slack, then the takeup with very little movement, then the break and little overtravel. Many rifle competitors (e.g., CMP, service rifle) use two-stage triggers.
The stock AR15 trigger is usually 6+ lbs and can't be easily, reliably, and durably improved without replacement.
"Lazy?" Or overruled by that meddling bean-counter, Robert MacNamara?
You know, the firearms genius who decided a chromed chamber and barrel was a luxury an infantryman's primary weapon didn't need.......