Whoa!

I know this to be a most knowledgeable and helpful forum, but still I am astounded at the quick and detailed responses.
Thank you all!!

A BIG thank you to kuduae especially - your information was precisely what I was seeking!

Frankly, I feel a bit stupid asking about firing a shorter round in a longer chamber. I should, of course, have realized the headspacing isssue. In my defence I shoot rimmed cartridges....

You are also right in that the Sako barrels were for the smaller diameter I bullet, not the S bullet which further complicates things quite a bit.
There was some discussion on the Sako collectors' forum as to the reason for this. After all, the rifles were manufactured from 1951 onwards, and the other 8mm calibers the gun was chambered for were the 8x57IS and 8x68S - why the smaller bullet 8x60 then? Apparently, if I wish to buy such a rifle slugging the barrel will be quite important, indeed.

Once again a big thank you!

- Lars/Finland