These are a nice little rifle to shoot.
I have put all manner of surplus ammo through mine, not to mention various and sundry handloads. Never any problems
It was designed for the 7.9x57 military cartridge and that is wjhat it was used with for many years. The 1904 version of this round had a 154-grain pointed flat-base bullet that came out of a Gew 98 at almost 2900 ft/sec, which is anything BUT a wimpy load. In the latter partof the Great War, German machine-gunners were issued with an anti-Tank loading which used a 196-grain pointed boat-tailed bullet at something close to 2700 ft/sec. This was very steamy stuff, likely higher pressures than anything loaded today, and I am very sure that a lot of it got run through those nice little Kar 98a rifles.
Stay relatively sane in the pressure department and, if the rifle headspaces correctly, no reason at all not to shoot it, apart from "value".