Here is how it works...... Your barrel slugs at .427 and hopefully that is the dimension at the groves. Your cylinder throats are a nominal .4235. That is .0045 smaller than the bore. So ......... with the bullet swaged ...0045 smaller than bore, the bullets are slipping and sliding around down the bore, leaving lead and really not stabilizing.
Next, with the small throats and most common bullets, the swaging process starts at the throat. So for a nanosecond ore three the bullet actually becomes and obstruction and than squirts out down the bore. Pressure goes up and more importantly, so does "Felt Recoil." The biggest biggie is the Felt Recoil. The elevated pressure isn't enough to present a danger.
In conclusion (thought I'd never shut up didja), your chamber throats need to be opened to .4275 or .428. I'd personally opt for .428.
Coffinmaker
PS; Then I'd order my bullets at .428