Gun Parts Corp. has 99 Savage .358 factory barrels. Is there any changes necessary to the magazine on the 99 to convert it,say, from a .300 Savage to .358?

I suspect that you can get away with a change from .308 to .358....or from .243....However I'd not attempt to change a .300 to .358. I believe there was metalurgical differences in the actions.

Hacksawtom, If your .300 SVG. is less than 900,000 serial # you don't want to go there. Provided cost is no object, and your 'smith is well provided with machine tools, plus a lot of machinist ability you could get away with it.
1, Savage switched from a square thread to vee type generally at that serial threshold. This number is not chisled in stone. 2, The .358 magazine rotor(spool) is not the same. 3, The magazine follower/ctg. guide is different. The older one pivoted from the center shaft of the rotor at the rear.4, The back wall of the feed well "tunnel" was removed and replaced with a cup like affair which formed the magazine rear wall and provided a bearing for the rotor shaft. This to provide added length to the magazine and make machining less costly. 5, Some time in the '50 s the breeching surface of the breech block in the receiver became flame hardened- to the consternation of those who attempted to drill and tap for the Redfield or Weaver bases. The older receivers were perfectly adequate for factory ammo but Savage believed that handloading was a coming thing and prepared for the "Happy Handloader" and his ballistic experiments. At my behest my father bought a 358 serial around 918,xxx. He later decided that the recoil of the 358 was a bit much for him and brought it to me with a Vee thread 243 barrel. The barrel was secured and sent from the Authorised Repair depot because I was still on their list. 6 months later and well past that hunting season Savage relented,actually researched their own records and even better found another 243 barrel-this time with square threads, but not without pictures and nearly forensic style notorised statements leading up to that point. The take-off barrel was in the estate inventory of the dealer tagged with my name and some of the paperwork. Most of the collector types didn't want to believe that a 358 barrel could have a square thread, and I received several phone calls about it.
Tom Burgess

The real trick is finding a spool for the 358, the 300 one wont work and Numrich (last time I checked) has long since run out of 358 spools.

If you want to get more from your 300 get some 308 milsurp brass and size it to 300s, that gets rid of the case head seperation problem. Then try some Rlr 15 behind 165 grainers.

hmm, interesting...

I don't know that the 308 based cases will feed through teh 300 based action, without mods.

if you are deadset on this, i would suggest 2 options...

1: trade for a 358 99
2: trade for a 243/208 99 and rebarrel

then you should have no problems
jeffe