The book says 869.99. But the book can sometimes be on the low side. The fact that it is old and minty makes it more sought after. I lloked on auction arms and gun broker and they are asking from between 775 and 2300 bucks for different 40 xb'x. but none like yours are listed. But asking and getting are two different things.
I am a gun show junkie and my guess is around a grand. But the unusual caliber may make harder to sell. Then again since it is unusual and as such harder to find, there may be some guy out there who has been looking for that very rifle for years.
Sounds like a beautiful gun. Your grandad had good taste.
Have you fired it? If it is indeed unfired, then it is worth more unfired. But if it has been fired the value drops a little. A piece like that was meant to be shot.
Are you trying to sell it or just value it? It would be ashame to sell it being you grandad's old gun. And I bet it will turn out to be ludicrouly accurate.
Can you post a pic?
okcmco