It can be opened up for the Lott. Your gunsmith will have to lengthen the magazine and massage the feed rails & ramp. My .300 H&H was built on a .458 length Mark X action and that is what my 'smith did. It is now wearing a .375 H&H barrel. I have been shooting that gun for 20 years or so opened up and it has never given me any problems. One thing I would recommend is having a gunsmith who has built dangerous gane rifles do your work.