fullofpaint is correct (though some claim them to be walkable). The ULE Trigger pull is nice because you get control over the reactivity of your mag. With fewer shims it becomes less reactive, but with 3-4 shims it can be downright scary fast. adding more shims has a similar effect to shortening your on/off pin. They are also great for pneumags so you can run lower pressure on the LPR & Ram.