Let's suppose.....
Take a Mini-14, M-14, whatever, barrelled action.
Take the upper assembly, and modify the receiver so that the trigger guard would no longer latch to the receiver, or fit the original stock. This would make it impossible for the barrelled action to be assembled and fired without a stock.
Create a new lower receiver incorporating the fire control group, magazine housing and ergonomic features (the portions that contact the shooter - grip, buttplate, forend/foregrip, etc). Perhaps the new lower receiver could be attached using screws into holes drilled and tapped into the upper.
This new lower would most definitely not function with a standard upper. It is therefore not a conversion stock into which a standard barrelled action could be installed.
Apart from the uberkewlness of such an assembly, how useful would it be? Think in terms of a M-14 slamfire occurring under one's face.....
Just a thought.....