gunnut, I spent years shooting IHMSA and experimented with every crimp, powder and powder charge, sizing methods, expanding methods and primers. I found a firm crimp is all that is needed to hold boolits in. To go overboard and crimp with a full profile like the factory .454's just shortens case life. I expect 20 or more reloads before replacing brass. Some that I gave away are still going strong after an additional 20 to 30 loadings. I tried no crimp, shooting one at a time all the way to full profile, shooting with a scope from the bench. In every case, the use of magnum primers enlarged the groups.
I see you use the same load I do. Have you ever tried standard primers? Or do you just believe the blather in the loading manuals?
By the way, I shot a pile of 39's and 40's with my Blackhawk and won Ohio state with 79 out of 80. With my scoped Super Redhawk I was able to hit beer cans at 200 yds from the bench. With the Blackhawk I could hit beer cans all day with open sights at 100 meters from the Creedmore position. With the SRH I kept 12 out of 12 shots in a 5 gallon can at 400 meters. If you are getting this accuracy with magnum primers, stay with them. I won't.
I have also shot many one inch groups at 50 and 75 yds with my .45 Vaquero, same load as yours except I use the LBT 335 gr. WLNGC and STANDARD PRIMERS. If you have not tried them, you should not say it is wrong!