Jim,
Which insert are you using on your bump gauge? I use the one in the video with the .400" insert, and get 1.625" on my 260 fired brass, and resize to 1.623". If you're using the same insert, and getting 1.566", that's a pretty big difference. SAAMI specs are 1.630" - 1.640". As for your charge, I am WAY above 39gr with a 130gr bullet and am fine with pressure. I don't think that's your issue. To get a case separation, it's almost always from over bumping the shoulder, but having a hard time nailing it down. I would hate to think it's specific to the Lapua brass.
Which insert are you using on your bump gauge? I use the one in the video with the .400" insert, and get 1.625" on my 260 fired brass, and resize to 1.623". If you're using the same insert, and getting 1.566", that's a pretty big difference. SAAMI specs are 1.630" - 1.640". As for your charge, I am WAY above 39gr with a 130gr bullet and am fine with pressure. I don't think that's your issue. To get a case separation, it's almost always from over bumping the shoulder, but having a hard time nailing it down. I would hate to think it's specific to the Lapua brass.