Everyone's experience will vary, most of my headspace failures were 98k (none ever failed the field gauge), the only 98a issues I have experienced were with stuck bolts after firing, one rifle would lock up like a vise, but after some firing and cleanings it lessened in severity.
It is always best to be sure when dealing in headspace, so do not take the chance, but typically these service rifles one encounter, that weren't abused by subsequent service (Albania, Turkey etc..), rarely have serious issues in my limited experience, but I never bought junk rifle either, except Turked G98's I bought to part out.
Interchangeability was certainly possible in Germany at the time, especially at the Loewe group owned factories (DWM, Mauser) and quality of production was superior at both firms compared to the arsenals (especially the Prussian), but old traditions die hard, guilds and skilled craftsmanship were strong traditions in Germany and while the US embraced the newer methods faster (
the pressures of mass production, in the US labor was more a problem, Germany never had the labor unrest or wage pressures the US had... firms like Loewe were models of the future in regards to dealing with labor, safety, compensation, worker facilities etc.. the US trade journals often used Loewe as a European model for such advances... then there was the differences in doctrine between the US/England/Germany in manufacturing, - the US focused on simplicity, durability, mass production capable of operated by less skilled men; England on extreme quality and multipurpose machine tools run by high skilled/cost labor; Germany a middle approach taking from both for their own markets, domestic and eastern European focused, - Russia a major customer throughout the late 19th century through 1933) Germany was conflicted in its approach, especially at the sluggish and unimaginative arsenals where the stolid and dull workers found employment (
Loewe had a reputation for luring all the best workers away from the arsenals, the best pay, facilities, benefits, but demand the best performance)
In general interchangeability is not a big issue with G98's, more a problem with 98a, especially stocks, - almost no issue at all on the 98k, but not quite to the level of US WWII production which was next to 100% interchangeability. Ernie Pyle wrote a lengthy article about an mobile Army Ordnance shop working behind the lines on Garands, naturally he spends a great deal of time on the men's personality, but he delves into the simplicity and durability of the designed that allowed such simple work based upon interchangeability.
Both my old Gew 98 and this one have backed out primers. Never checked the other one, but this is common in bringbacks. In WW2, you had a pile of rifles, and a pile of bolts. One rifle, one bolt, then off to fill out the paperwork to bring it home.
As to the other issue, I had always assumed 100% parts interchangability was standard for all military rifles. Disassemble 10 1903s, mix the parts, reassemble them and all 10 would work. Ordinance sent word back from France in 1918 to quit sending Winchester 1917s because they only had 95% parts interchangability .
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