I have to bring this up, it's from 2006 when I contacted Diamond about some work.
I had some corrospondence with them (they answered my inquiries quickly and courteously) about having some work done to my 2-1/2" chambered Flues 20ga. and it came out that, quote,
"I won't let the gun leave my shop with short chambers. Liability reasons. To lengthen chambers is $30.00/barrel. I'll test fire with 2 3/4" light field loads."
I didn't want the chambers on my gun lengthened so I passed.
Just a heads up if you are considering having work done, I'm not sure if chamber lengthening is still mandatory so you should call first and get it cleared up. Hate to see someone's original gun be altered unintentionally.