There are three different 8x57 German military cartridges used for general issue. The first used a .318 roundnosed 226 gr bullet @ 2100 fps and about 40k psi (SAAMI recommends only 35000 psi and 37500 Cup for these btw), the second was used through WWI which was the 1905 spitzer flat based bullet of about .323 spitzer 154 gr.bullet @ 2880 fps. The WWII issued 8x57 was used in light machine guns as well as k98 rifles with the spitzer 197.5 or thereabouts gr. boat tailed bullet @ 2500-2600 fps fps.
http://www.omegacrossroads.com/GunCabinet/8X57/8mmMauser.htm
Older THR thread about ballistics of 8x57 Mauser
https://www.thehighroad.org/index.php?threads/8mm-mauser-ballistics.227273/
Be careful about applying Euro pressure stds. by CIP (Euro counterpart to SAAMI) to surplus rifles in the US as European firearms generally require proof testing, and in some cases require re-proofing firearms that are sold to another or substantially altered--new chambering, etc.
The GEW 88 Commission rifle has a Mannlicher style split bridge at the rear and the subsequent thumbcut in the 05 alterations did not do receiver strength any favors. It does have front locking lugs on the removable bolt head. The original issue of this rifle using the .318 bore is one of the reasons that pressure levels are set low by SAAMI for this cartridge as many were imported in the States and I believe that German sporters used the .318 bore size a bit longer than the German military did from what I have read. Unless the barrel is stamped S for spitzer for a .323 bore adopted by the German military in 1905, then it has a J (I) bore of .318. A lot of markings have been scrubbed or are unmarked from sporter conversions so slug the bore if you get one.
Re: 8x57 use in m98 Mausers types.
Obviously the later m98 model WWII rifles were generally built with modern metallurgy which allowed them a bit more leeway pressure-wise but the earlier GEW 98's are generally acknowledged as a bit more variable in their construction with a fair amount of soft receivers. Late WWI era GEW 98's demonstrate more variability in materials and workmanship as wartime production quality slipped--starvation, cold, wartime news, labor issues, and deprivation seems to have affected worker motivations and material shortages were everywhere.
Similarly, late war k98's (1944-45) vary a lot in production quality and there are persistent rumors about slave workers and forced labor motivation and sabotage problems in some plants. Use of such slave and forced labor practices in producing k98's is murky except for surviving documentation of certain factories, such as Steyr and Gustoff Werke definitely using it. But overall from surviving reports, we do know that slave labor use began slowly in 1942 but rapidly increased to making more and more of Germany's war time production by 1944-45.
Germany's manpower was being shoveled into the Army due to casualties on the Russian Front and elsewhere and able bodied males were scarce. Also, bombings and material/tooling shortages resulted in a rougher and rougher product and shortcuts were made in quality control to simply have firearms to shoot. Of the wartime makes, Oberndorf and Brno seem to have kept quality standards a bit more and were in relatively safe areas not affected by Allied bombing (latewar codes byf and dot). Have no information about slave labor use by those manufacturers.