I'd fire it with some lighter .45 Colt loads and see if you get the double firing pin indentations. If you don't, then it's caused by the recoil of the .454 overcoming the hammer spring. I'd also try it with some factory ammunition, too.
You might give Ruger customer service a call, but they're just tell you to send it in so they can check it. They may give you a mailing label, though, and pick up the shipping charges. It might be worth the call. They won't like the fact you're shooting reloads, so you might not want to mention that if you call.
Hope this helps.
Fred