Are you getting light primer strikes using your G21?I converted a Glock 21 to 460 Rowland. Ya its a cannon alright. Its on the lower end of a 44 mag. 13+1 Rounds. Reloading a few 10ths of a grain off the max load published by 460rowland.com, I'm getting 230 grain xtps going about 1320-1330 fps. The only problem is a revolver goes bang every time you pull the trigger. I am not getting that with the 460 Rowland. Maybe it needs a few hundred more rounds through it to break in.
On a side note I am getting 100% reliability out of 1 of my Glock 20s (10mm) which has about 600 rounds through it now. I will be carrying that as a side arm hunting this fall. 180 grain bullet going 1300 fps.