Marlin makes all of H&R's barrels, has been since 2007, H&R will be making their own barrels later in 2010, until then Marlin is making them and they have their own barrel making quotas to meet, they work H&R's in as they can, that's the main reason why H&R will be making their own barrels, H&R hasn't got the centerfire rifle barrel production schedule from Marlin yet, except for the Huntsman which is scheduled for the first quarter 2010, I'll post when they get an updated schedule.
Matt, my H&R contact has stated they will fit 357 and 44 mag barrels to those frames, I don't recollect anyone stating they got one, probably because they may not know, I do know that one person was refused a 45-70 barrel, that's how we first learned about it.
Tim