A friend of mine spent a lot of time (and money) being fitted by Brian Bulinski,(Fieldsport) in Traverse City,MI. He was very happy with the outcome. He ended up with two sets of dimensions - one for O/U, and one for SxS. Apparently has to do with barrel "flip".
Subsequently, he went to Kolar, and ordered a new gun. they again spent lots of time with him, and the results were alomost identical to his prior fitting. His particular problem is that he needs a great deal of cast, and can't get it in a production gun. Once he got the dimensions from Kolar,and selected his stock blank, it took about a half hour to have it turned out on their CNC machine. I've seen the gun - they did a good job
