I have a RRA mid-length carbine that I got new as an HBAR in 2006. Way too heavy and not balanced the way I wanted it and so I sent it to ADCO in 2008 to have Steve re-profile the barrel to a lightweight and shorten it to 14.7". He pinned and welded the A2 FH to it so it was 16.1" and legal. The work took almost 1½ lbs off the weight. I didn't change any springs or buffers.
I sent them a new front sight with a .675" hole (vs the std .750") so they could reduce the barrel under the sight as well. Steve gave it a target crown and opened up the gas port just slightly and it's worked slick as snot ever since. It's more accurate than it was when new and I can still shoot it out to 300yds at bowling pins.
Stock HBAR barrel:
My re-profiled and shortened one back from ADCO:
For grins, I added an Enidine buffer a couple years later and it still hasn't missed a beat. Weight and balance is great now and it's my lightweight irons gun. At 5lbs 15oz it's a great carbine.