After a most frustrating attempt to wade through federal firearms regulations and a need to build a .70 rifle, are there legal ramifications? Is larger than .50 a 'destructive device'?

It is confusing and the answer is yes and no. There are grandfathered cartridges like the old recognized British .577NE and .600NE that are exempt by regulation. State laws also come into effect here. For example california doesn't allow over .60 cartridge guns except for 12 Ga.
There are registered cartridges like the .700NE .700HE, 550 magnum and .600OK that are exempt by registration( ED, Neal and I have gone this route). All 12 Ga .729 shotguns and slug guns as well as all blackpowder rifles and cannon. Thats why the 12GaFH is perfectly legal everywhere! I've been told this also applies to 10bores,8bores,4 bores and 2 bores( but never confirmed it in writing with BATFE. There may be more I'm not aware of. So its basically a named cartridge,type ( shotgun) or registered as a sporting cartridge. For example, no one knows if the .585 Nyati is or isnt a DD! A .700 on any case using smokeless powder will be a DD unless you go through the process of registering it and have the subsequent gun you build inspected by BATFE. Bottom line is if its not specifically in the regulations, better have a letter from BATFE saying its registered or covered!
Play it safe and make sure your covered.-Rob

Thanks for the excellent info. I want to build a 70-150 on a 87 winchester copy as a matter of history. I know others have talked about this but do not know anyone who has actually done it. Perhaps they ran into the bigger than 50 problem and did not continue.

If its designed for blackpowder you should be exempt, however if you are designing for smokeless you might have a problem. I'd write BATFE and explain your 70-150 based on a Blackpowder case and see if they deceide its exempt or not. I would not do anything until I got the answer in writing though. Don't take anyones advice, get the facts.-Rob

I have received word from BATF that it is ok to build the 70-150 as the cartridge was designed for sporting purposes. It is also ok to place it on the 1887 winchester frame as that firearm was designed for sporting purposes. Very Good!
Anyone got an extra .70 barrel blank?

Thanks for freely giving out this advise, Rob. It could save a lot of grief.

It is ok to put it in an 1887 Win levergun
as it, 70-150, is a grandfathered in case,
and Coyote Cap already does this as well as
12ga in moderen steel replica actions that they
import. I have 12ga one, and I did
extra barrels in variety of cases.Ed