Simply put, I have an M16 A2 clone and it has a DPMS HBAR barrel on it now.  This was the only 20 inch barrel I could find when I built it back in 2013ish.  I want to replace it with a government profile barrel and figured it would be cheaper to just have mine reprofiled to government spec under the handguards.  It's a good barrel and I've been happy with it.  Is there a shop or something in Missouri - especially in the west-central part of the state-that can do this?

It likely will be cheaper to buy a new barrel

+1

I'd go this route Model 1 sales.  They are gtg, as I have had great service from them for years... Machining long tubes induces lots of vibration.... just to say it's a pain in the butt....

My oldest AR still has a M1S upper and barrel on it still HBAR
They used to use ER Shaw barrels. Not sure if they still do. This was back around 2005.

Get a new one.  But seriously, if you must, stick with the tried and true like adco.