7,87mm is .3098", so a D166, wich measures to .309" is as perfect as one can get

There are alway exeptions to the rule, I have a M27 with a .309" groove, it hates D166 bullets but does wonders with Lapua D46.
If you have the "Visier" read the article on the finnish rifle(April or March 2004), I know the freak, who made the part of the load development

Chris