Very nice rifle and I hope to own one some day myself! As far as loaded ammo, it looks like Buffalo Bore also loads for the fine .348, but as with most of their stuff, it ain't cheap! Another bullet maker that seems to have several bullets for that oddball size is Hawk Precision Bullets (9 different bullets):
http://www.hawkbullets.com/Pricelist.htm
Added: While looking at some other loads, I noticed that Stars & Stripes Ammunition also loads for the .348. They have loads with up to a 270gr bullet (Hawk?)
http://www.starsandstripesammo.com/index2.html
As far as the Improved version, I'd think the original to be powerful enough at the ranges it is practical for on anything short of big bears. Range is somewhat restricted as scoping a M71 is usually not something most want to pursue. The M71 died originally because Winchester brought out the .358 Win a few years earlier and figured the easy to scope M88 in .358 was a better platform for most hunters. If one was wanting to hunt big bears at close range then a conversion to .450 Alaskan would prolly be a better bet, I'd think.
If I am lucky enough to finally get one, I'll simply have to limit my shots to whatever these old eyes will do with a peep sight and at those ranges I figure the original .348 to be plenty for my needs.
Good luck with that great rifle!
