I currently load these rounds with A2400, with good results in barrels in 1:20 to 1:14 twists.  I am working on a project to line two barrels for my Winder low wall in both calibers.  The liners will both 1/2" with a 1:10 twist, so I can load longer and/or heavier bullets.  I would like any ideas/experience you have with powders for these cartridges that takes advantage of the longer (29") barrel, with similar velocity of the A2400, but lower chamber pressure.  Any thoughts are appreciated.