>Pushing a rifling button through a tube with a carjack
>difficult
You can source precision moly steel tubes anywhere on the planet. Lap the inner bore on a fixed rig (linear lapping should be ok), push the button, thread the barrel and call it a day.
>You probably could have double stack in your cartridge but i doubt that'd go well as 10mm double stack has had complaints about its size and i'd consider it the upper limit
That complain is absolutely stupid. It is only an issue for women and manlets, not a very relevant part of the market. The main issue was handling the round on a pistol with slightly-bigger-than-usual grips, the root cause of the problem was poor grip of the firearm. If you ever get the chance, compare Glock 17 grips to bigger grip glocks (the .45 or 10mm), the difference is not significant.
The chinks made double stack 7.62x25 clones of a SIG 226. Perfectly doable.