Thank you for the tips so far!
FYI I don't know what Hornady or Winchester use for posted velocities with the factory ammo. For the Sierra handloads the manual says they used a Blackhawk with a 7.5" barrel. The 300 Sierras were loaded at 1.715 OAL, just a touch shorter than the books 1.735. I doubt the extra 200 fps is coming from the extra .020" squish.
Due to the fact that the extra velocity is present with factory loads make me less concerned that the handloads are dangerous - I believe that most stock ammo is loaded with a certain modesty.
I first suspected the chrony but I did check it againts two different factory loads of CCI 22LR from a 10-22. I checked the 44 on two differnt days at two different muzzle distances. I highly doubt the chrony is out.
I think for the 300 grainers I am going to stick with 18.0 grains (a safety margin below the posted max 19.0) and live with the extra speed.