I don't know enough about the low mass carriers but I do know they are part of a system, a call to JP or whoever might be worth the time to see exactly what you need.
Another way is VLTOR's A5 system, you can buffer it how you need it and using the rifle length tube and spring is a lot smoother than any carbine. It's probably cheaper too.
On the no go gauges, all you need is a field gauge to check an assembled barrel.
