Not to mention the time needed for the different manufacturers tooling up for a completely new product, and before that could be done there had to be set some standard for the minimum and maximum matrix on the tools, plus the norm for a standard load.
The task is quite simple when you're making loads for one rifle (your personal) only back home, a completely different story if you're supposed to resupply an entire army issued weapons from several different manufacturers.
Oh, and the idea of a 135 grain bullet instead of a 110 grain in the M1 Carbine makes sense if you want to make a subsonic load.
Given the fixed length of both magazine and action on the M1 Carbine the heavier bullet would have to be seated deeper in the case, decreasing the alotted room for powder, and consequently the charge would have to be reduced in order to keep the preassure down at an acceptable level.
Some guys have tried to make 6 mm. bottlenecked wildcats based on the M1 Carbine round, but the single locking lug on the M1 Carbine doesn't cope well with the increased preassure.
Today we are all Norwegians! Swedish Foreign Minister Carl Bildt. 22. July 2011