Just because I could, I loaded a Ruger SR40 mag and inserted it into my PCC ( Ruger magwell installed).
The mag locks in place and the rounds are stripped from the mag and pop out the front of the receiver when I cycle the action with the barrel removed.
With the barrel in place, the round enters the chamber till the diameter of the lead is too large for the breach.

Looks like a .40 conversion may not be too difficult.

My guess is that Ruger was:

1) waiting to see how popular the platform would be prior to investing in additional tooling

Or

2) is waiting to see what is produced in the aftermarket, allowing others to eat some of the R&D costs

As an example of option 2, see CZ's collaboration with Manticore.  Success of such partnerships may pave the way to the future of modern arms design and manufacture.

From all I can see, the PC Carbine has been wildly popular and is a viable option for some ban states.  I would be shocked if you didn't get your wish in the next year.

Need 10mm conversion.