I'm not really sure what 5R rifling is but it sounds like something I'd want The trigger is cheap. My AR556 came with all MagPul MOE furniture as well. No doubt if someone was buying today that this would be the way to go. I guess I'm mostly convincing myself I don't need another
The trigger is about $100.
Here's a pretty good explanation on 5R rifling. It makes the gun slightly more accurate and easier to clean
http://faxonfirearms.com/blog/what-and-why-5r-rifling/