I like my H&R 45Colt/410 with the 3" shells but haven't yet had a chance to try out the .45 Colt loads. The un-rifled "throat" of the chamber is long and oversized for the bullet anyway up to the start of the rifling so all efficiency and accuracy theory are against you from the start but sometimes surprises and contradiction to convention do happen. But unless you reload your 45 Colts I think you may have to accept less accuracy than you might like.
But I do think that a lot of folks (present company excepted, of course) are prone to making blanket statements on woeful performance based primarily on theory and sometimes without any trials at all. If you decide to try some standard 45 Colt factory loads if any or made with jackets, I'd start there. If you reload, some heavy jacketed bullets at about 900 fps would be worth a try, I think. I don't think I've seen a posting/thread/link here that demonstrated a diversity of attempts at shooting the 45 Colt in this firearm. If you cast, you might want to give a slow, heavy, gaschecked SWC, cast from a kinda soft alloy, a try out.
I ordered mine drilled and tapped from the factory folks about a year ago when I sent a receiver in for fitting to that barrel and they seem to have done a good job. My health got bad about the time I got it back and haven't done as much with it as I'd like to have done. Any findings you get from shooting yours would be valuable for anyone shooting this firearm, I think.
stuffit