First off, there isn't any need to purchase an expensive piston nut wrench for a one off job.
There are two options, easily available to anyone with a file or grinder or both.
Use a cast off socket, of appropriate size and make up a castle type tip to fit those of the piston nut.
Even a regular hand wrench can be ground to fit opposing sides and do the job well.
You can even use a very soft punch, along with a light hammer to coax the nut into position to accept the retaining pin.
This doesn't have to be complicated.
OP, PM me about an NR 30 M1 NR barrel. I have one on hand and I can't see me using it.
If you don't want to purchase mine, which won't be cheap, your existing barrel can be sleeved from the chamber to the length you desire.
I have one, that I sleeved with a Shilen take off match grade barrel, which is finished at 21 inches. It shoots well.
Sleeving M1 Carbine barrels is nothing new and if you looki it up on the internet, Universal actually did just that with surplus US barrels that had been cut off just in front of the piston/port.
The after market barrels need to be reamed for proper headspace and timed for top dead center, or they won't operate properly and bind.