Like the title says. Were there any problems in fitting? What about dimensions? Who did your machine work?

You'd probably be better off getting an aftermarket barrel specifically made for use with O-rings like Capital City Firearms barrels. You just need to polish up the inside of the opening at the muzzle where the barrel pokes through and you should be good to go. You might just want a threaded one while you're at it for the coolness factor or possibility of using a flash/sound suppressor.

I've switched my USP9SD over to an o-ringed Jarvis extended so I could thread it for my 9mm Phantom suppressor.  Really tightened up the groups.  The SD was probably my least accurate HK before the barrel swap.  Now it's right up there close to the Tactical.  I still need to get the trigger swapped for a match......

I've got a CCFA Barrel, as well. Cant recommend it more highly.

Doc @ M&A Parts

Larry at CCF tried cutting the o-ring groove on HK barrels and wound up destroying the barrels, and then having to replace them (ouch!).  The HK barrels do not lathe well (hardness problem IIRC) and Larry does not recommend cutting the o-ring groove in them, and will not do it any more.  Either go with a CCF barrel or a Jarvis barrel if you want the o-ring.

My USP9SD also is the worst shooting HK I own.  Even my Compacts shoot rings around it.  The only way I could get groups smaller than 2" from a rest at 25 yards (when the Tac and Compacts easily go 1.5 inches or less) was to shoot 147 gr. bullets and use Larry's Titanium flash suppressor.  That pistol royally sucks.  Since I am not a fan of the 9mm anyway, that pistol is retired.  I bought it mainly for the collection and if not for that I would dump that pistol in a heartbeat.  HK never should have compromised on that abomination.