The Atrius FRS is designed to work with mil-spec AR-15 fire control groups like the Colt M4 6920, but compatibility with a full-auto BCG specifically is not something I can confirm from the available product information, so I am not sure.
This selector only enhances semi-automatic firing speed through its FULL-SEMI mode; it does not convert your AR-15 to true full-auto, and no further modification for full-auto function is described or supported.
This is a complete, standalone Forced Reset Selector (FRS), not an FRT, so no additional parts are needed to install and use it; it drops in to replace your factory safety selector and works with mil-spec fire control groups as-is.
The OPMOD Atrius Ambidextrous Forced Reset Selector is a 3-position (SAFE, SEMI, FULL-SEMI) selector designed to drop into standard mil-spec AR-15 fire control groups without modification, so it should work as your 3-position upgrade.
I am not sure, as the listing only confirms compatibility with mil-spec/full-profile AR-15 fire control groups and does not address specific brands like Nexus or KUNA.
Yes. This will fit in an standard Mil-Spec AR15.
The OPMOD Atrius Ambidextrious Forced Reset Selector requires a full-profile BCG. This means you will need to purchase an M16 style or full-auto style BCG.
This will work with any standard AR-15 lower, regardless of caliber. This is assuming you have the correct buffer, buffer spring, etc.
This is designed to work with any standard AR15 lower.
This will work with any standard AR15 lower and trigger.