The Enhanced Magazine Release-Continuous Serrations features a slightly curved button surface to fit the natural curvature of your finger. The form fitting curve, along with horizontal serrations, create a self centering surface that inhibits the finger's vertical movements. The EMR-C has an uninterrupted surface and serrations, its taller profile makes it easier to press and is only 0.04 taller than the factory magazine release button. When installed, the EMR-C has a 0.08 protrusion above the mag release button fence on most forged receivers.
Due to the EMR-C's installed height which protrudes 0.08" over the mag release fence when installed (only 0.06" or 0.07" of the magazine release button travel is needed to drop a magazine), EMR-C is recommended for 7.62mm ARs only, though it does work on 5.56mm ARs and M16s. For defense and duty ARs and M16s, it is recommended to use the EMR that has only a 0.01" protrusion over most forged receiver magazine release fence.
For .308 ARs, the EMR-C's taller profile will allow you to use AR15/M16 spec mag catch and ambidextrous mag catch/release. An EMR-C installed on a .308 AR will not protrude over the mag release fence.
Please visit the EMR vs EMR-C page for an in depth review of the differences between the EMR and EMR-C.