: The blown .348 case (.50 ALaskan) is usually taken as being the largest that will function in a Marlin, with an overall length of around 2.55" or a tiny bit more. It has a .545" head size, with a rim the same size as the .45/70. This is quite large with increased capacity, compared to the .505" head size of the .45/70 case.
: If by some stroke of machining, the Marlin will safely take an even larger case body, with it necked down to .50 cal, it may be possible to improve on these very worth-while ballistics.
: The trouble with 600gr. bullets in that .50 Alaskan case, is they take up available powder space, and since the rifle itself is restrcited to about 43,000CUP max, the 600gr. might get up to 1,800fps, perhaps not faster.
: At that rate, the FPE would run 4,314fps. Now, 1,800fps with a 600gr. bulet is certianly more than needed for most North American game, and most African, as well.
: With a 550gr. bullet, a velocity of 1,900fps might be attainable with the proper powder, but again, the bullets take up a lot of space within the Marlin's length restrictions. 550gr. at 1,900fps runs 4,406 fpe, which is quite incredible for a lever gun. This is why Harold Johnson liked the .50 Alaskan so much.
: If .348 cases are used, they are very easily formed, by opening up the necks to .375 and chamfering. .50 cal bullets, perhaps the 550gr., is chucked in a lathe, and 1/2 to 3/4 of the bullet's length is easily turned to .375". This is seated in the case over about 40 or 45gr. H4895, rest of the csse filled with plastic shot buffer and fired. The bullet's full dia. front end, seals the bore, while the light charge fo powder expands the case to full size. They are then trimmed to length, and you are ready to load full power loads. This 1/2 reduced bullet is the same method Jack Lott first used for forming his .458 Lott cases from .375 H&H mag cases.
: Lyman makes a 520gr. flat nosed bullet mould, and RCBS makes one that comes out at 550gr. in WW metal. Both are accurate bullets. NEI will have heavier bullet moulds available.