The Northland Shooters Supply Taper-Loc system allows you to install Rem/Age pre-fits without making any permanent modifications to the receiver.
http://northlandshooterssupply.com/nss-releases-new-tool-for-installation-of-remage-barrel-on-remington-700-action-without-gunsmithing/
Just to reiterate what a few other posts had mentioned... Velocity can increase if ammunition is left in the sun, though H1000 is relatively temperature insensitive. Ball powder can increase velocity if chambered in a warm barrel as well. Check to see if there is any copper fouling and if so...
We occasionally add reamers as demand increases for particular configurations (we track the number of requests received for reamer options not currently inventoried).
The nature of the pre-fit system may preclude it from safely accommodating some cartridges (specifically those with large...
+1 for the Criterion option, although I may be just a little biased :D
That said, I can assure you if you take the Criterion route we'll get you a shooter. In the extremely unlikely case it doesn't prove to meet your expectation, we'll troubleshoot, inspect, repair, or replace the barrel until...
Northland Shooters Supply should be able to accommodate requests for different contours, lengths, chamber configurations, etc. if OP is looking for something similar to the McRee models.
I'm not entirely sure why Fred says it would take 6-8 months to get one of our blanks. Current lead time on uncontoured blanks is 3-4 weeks, and uncontoured blanks are about 6-8 weeks out. Complete pre-fit barrels are currently averaging 12 weeks, with a quoted 14-16 week delivery timeframe.