I am shooting a lbt 325 wfn with a gas check over h4227, 2400, or 296 generally around 1100 fps.
if that bullet is really doing around 1100 fps' then i'd think that your "20 BHN'' is too hard to allow the bullet to obturate when it passes through the forcing cone. the working pressure and acceleration of the .45 Long Colt probably call for a softer bullet. (though in the .44 mag' a 20 BHN could be proper.)
your .4525" diameter for the cylinder throats may or may not also be the problem. as the other gentleman has said, throat diameters of the cylinders can be problematic. what is the diameter of the grooves in the barrel as well?
i'd also recommend that you use a two-hand hold with your weak-side hand setting lightly on the rest and see if that helps. but no matter what else, you are shooting a heavy bullet that has a relatively long barrel-time affecting the recoil pulse. are you handling the recoil well?
www.leverguns.com has a host of articles that deal with hardcast bullets and their lubes, hardness, sizing, etc.etc. i recommend you look at the three differnt sets of articles and study them to see why i question the BHN you're using, etc.
let us know how you do.
ss'
ps: i admire your choice in handguns...........RUGER !