I have personally built RUM class chamberings using Lapua receivers. Some work as if they were made for the RUM, others had a few issues.
I have limited experience with Lazzeroni brass, mainly with the Tomahawk and Warbird. The Tomahawk brass was worthless. Early Warbird brass was very weak. Newer Warbird brass is pretty good strength wise but in MY opinion its very expensive for what you get. Especially when you consider that a round based on the 338 Lapua case and fireformed will get you exactly the same performance. I know that because my 300 AX will easily match the Warbird in every catagory and do so with a tick less powder and brass that is much less costly and also much stronger.
Back to the receiver. You say you can just open the bolt face up for the 408. I would make sure the manufacturer recommends this. I know I had a BAT model M 1.53"x8.5" single shot in shop a couple years ago. Now this receiver is the receiver I use mainly for building my 338 and 375 Allen Magnum which are based on the 408 CT. This receiver however had a lapua bolt face. The owner wanted a 338 AM so I figured no problem, just open up the bolt face.
Just by chance I had to order some more BAT receivers so I was talking with Bruce and he said DO NOT DO IT....... He said the Lapua and the Chey Tac bolt faced receivers have more differences then just bolt face and that the Lapua receiver WOULD not work well with the CT class chamberings. He never said there would be a strength issue but it sounded more like a function issue, I assume with extraction and ejection.
Anyway, just make sure that the manufacturer does not have a problem stepping up their receiver from Lapua to CT bolt face.
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Joel are you having an issue with that action? If I wanted to go with a lapua style case would this action be ok for that?
No issues with the action for the Lapua case, but like Kirby states... not for the .408 case.
Personally, if I were building on the lapua case, it would not be on the Montana action. But, hey, that's just my opinion...