dearmer
I have never worked with an oval bore double. I can say that the 450 No2 is a GREAT double rifle calibre, one of the best IMHO.

However my 1000 yard Match rifle has a polygon bore. It shoots great and is very easy to clean.

Heckler& Koch made full-auto guns with polygon bores and claimed much longer useful life.

I would test fire the double and see how she shoots.