Yes, I know it's blowback. I should have been more clear and I think I erased some clarity from an earlier draft of my post. By 'scale up' - I mean take the best features of the PC9 and build a new rifle. Those features would be, common magazine availability and take down feature. The PC9 is a handy little gun for these reasons and I get the impression it is quite popular in ban states because of its feature set.
The closest thing to what I am imagining would be an SCR. Where you can use an AR mag and take the upper and lower apart for easy transport.
I'm not sure how many folks have regularly had to tote fullsize weapons in and out of apartment complexes in highly populated, unfriendly, areas - but it's not fun. Plus, there isn't a state I've found yet, where transportation laws would find a disassembled long gun as being in violation of their transport laws.
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Apparently, someone took a BRN-180 upper and put it on an SCR lower. That allowed them to eliminate the bolt carrier and switch to an Magpul SGA stock. And it seems like a really good idea actually. For this to by Ban legal, the flash hider would have to be removed and threads cut off. But that's a simple thing compared to other things. If Fitelight would make these things in decent quantity, I'd probably try to get one in .300 Blackout for reasons I don't fully understand.