I guess I have to go against the crowd on this one, I have both old and new build revolvers, and while I love my Monsons, the new guns have much better finish. I also own several of the new 1911's and they are excellent guns, I have contacted the factory a couple times, once for repair on a 1911, which was handled lightning fast (returned to me in 5 days repaired) and a couple times for parts on both my older revolvers and the new 1911's. The staff was courtous and shipping was fast. I only have good things to say about the current company and their products. They can be hard to reach, don't use email, call them and leave a message, they will call you back. I do wish they would promote the revolver line more, but right now they are concentraring on the 1911 line, as that is where the proffit is.
In answer to the original question, the owner, Bob Serva, has indicated that there are currently no plans for a 500 caliber. (As well as no new caliber offerings any time soon- I was hoping for a 17)
It's kind of interesting, DW did consulting work with Smith & Wesson on the development of the 500, but have no plan to introduce them, I guess they want to stick with the "familiar" chamberings now & not invest in a lot of tooling for what may be a niche market.