Yes, it is true that the Schoenauer rotary magazines used in the Mannlicher Schoenauer models prior to M1924 are rather particular regarding cartridge profile and overall length. The spool is shaped to cradle cartridges designed specifically for it, but is open around its circumference. The magazine body and surrounding well are machined to provide additional guidance and support to the cartridges as they are fed by the spool. If cartridges of shorter or narrower profile at the 'nose' area are used they may become misaligned and jam into areas of the magazine body.

Note how nose of cartridge aligns with machined guide toward front of magazine body

Considering, however, that they were prototyped (M1900) and initially manufactured ('Greek' Y1903) as military arms I'm not so sure I'd count that as a flaw. Supply lines provided proper fitting ammunition and with such the Schoenauer magazine performs flawlessly every time.

Y 1903/14 Carbine

As long as supplies of proper fitting 6.5X54 MS were available, Y1903 Mannlicher Schoenauers did not jam, could be fed singly or charged with 'strippers' of five rounds, and were indeed highly regarded for their reliability. Other military rifles also had particular feeding habits. The highly successful Gewehr 88 'Commission Rifle', for example, was fed cartridges in en bloc 'clips' without which the rifles could not feed at all.

For the sportsman, again it can be said that as the 6.5X54, 9X56, 8X56, and 9.5X57 were Mannlicher Schoenauer proprietary cartridges that the cartridges themselves were part of the 'Mannlicher System' and fed flawlessly through their corresponding Schoenauer magazines. I doubt that many manufacturers of the early 20th century gave much consideration to what issues reloaders may have when proprietary cartridges go obsolete.

The problem arose with the M1924 and subsequent models as they used cartridges that were not proprietary to the Mannlicher Schoenauer. The issue was addressed. A retaining ring was added to encircle the spool of the M1924, and modified for later models, which held cartridges in place at or near the shoulder. This addition negated feed issues when cartridges of differing profiles are used.

Schoenauer magazines from; M1903, M1910, M1924, 'M1925' (High Velocity), M1952
Photo swiped from post of member Kuduae

7X57

For those with M1900, M1903, M1905, M1908, M1910 models who choose to use different projectiles, some have added carefully shaped shims carved of wood or plastic to the areas of the magazine well which need to be built up to fill voids left by shorter or narrower bullets. Not an easy fitment, and then your MS is cartridge specific to the 'new' profile.

As long as one uses cartridges built to original specifications, however, an original Mannlicher Schoenauer magazine simply will not jam.

Animation MS

I've never operated a Savage 99. My only Savage is (most of) a Savage built 1928A1 Thompson Submachine Gun. I'm not Class 3, so it's one of the parts kit Tommys that were imported to the U.S. in the 1990s - 2000s. The original upper receiver did not come with it, unissued lower and barrel are assembled to a dummy (inert) receiver.