MPA Mac Barrel length confusion

[Heavy_Grinder]

12133PC requires Roster-exempt single-shot pistols to have 6" min barrel length and 10.5" min overall length (measured parallel to bore).

Thinking about a buying a top cocking upper to compliment my side cocking MPA30SST-A. I have the option of buying the upper with a non threaded barrel which MPA states is 6 inches. If this is clearly legal, why not just make all Kalifornicated MPA mac's for sale in this configuration?

Will this be good to go, or just to play it safe, should I just go with a standard threaded barrel and have the usual Titan break permanently attached?

[Rekrab]

The barrel and overall length are, to the best of my knowledge, only needed for the transfer. After that you're pretty much free to do as you please provided you aren't breaking any assault weapon laws.