Re: 300 grain, .338 Matchking - True BC ??
I've looked at that case/bullet combo enough over the last 3+ years to know the junction of the boattail and body are probably right at the junction of the shoulder/neck of the case. If I had to guess I would say in this picture it might be just a little bit below. That's how Dave Tooley throats them usually.
Remember though this is a blown out case so the neck is approx. .050" shorter (base of the old neck is now in the shoulder) than a std Lapua case like your going to use. So that will work to your advantage some but your still going to have some of that 300gr down inside the case with the 3.625" OAL limitation.
Rough calculation would be 3.875-3.625=.250" into the case assuming all else is the same. Those 300gr bullet tips vary a lot so the 3.875 measurement will vary from round to round even if your seating depth is constant.
Hope this helps,