I had a very well known US gun firm do all the metal work for .375 Weatherby rifle for me from components that I supplied them. It was to be barreled, have sights installed, nice bottom metal, and a checkered bolt handle. One component was a wonderful Jim Wisner made tapered octagon barrel with integral rear rib and front ramp. The idea was that all the metal work would be completed, the rifle would be sent to my stock maker, and then returned to the big US firm for final sighting, polish, and blacking.

However, when the rifle got to the stocker, it arrived without the bolt knob replaced - it still had the original military straight knob. The stocker could not complete the stock without the bolt handle but I didn't want to send it back and wait another six months just for the bolt handle to be replaced. Thankfully, Jim Wisner offered to replace the bolt handle for me.

While he had the rifle, Jim also checked the headspace. He reported that it "barely" passed the no-go test and he was worried that after firing a relatively low number of rounds it would end up with excessive headspace. To remedy the situation he built up the bolt face with a steel wafer. I am not sure how he installed it but it was undetectable and it apparently worked like a charm. When the rifle went back for finishing I advised the firm of the change. Though disgruntled, they confirmed the headspacing as corrected by Jim was spot-on.

If you do find that your .375 Improved has excessive headspace, you might be able to get a similar fix as an alternative to setting the barrel back.