The weight thing is one of the criteria wich defines what is a good gun vs a bad gun.
We all know that pistols over a certain wight, particulary if you can hold them two handed (without using both hands on the grip) and they have the magazine in front of the trigger are particulary evil, and known for making unsuspecting owners rob banks, join criminal gangs ans mass murder other people...
so fortunately the carbon 97 pistol is defined aas a "good gun", and then using the carbon lower for a rra 9mm upper for training purposses is perfectly safe and legal...
Since colapsing stocks are known to make people do the same hienous things, i got a spare one from my Bushmaster VM 20" including the buffertube, springs and whatnots. SO i do have a regular carbine buffer set up to use with 9mm upper. the main question, wich i think have been answered is that with changing the buffer, a rra pistol upper would work and fit to a Bush carbon lower.
How about magazine blocks, like the Hahn block or the RRA magblock, would those fit?