You will likely get variable answers to the first question, but there may be some trends.
You are unlikely to get any real answers to the second question. Unless there is surviving original documentation from period testing that someone has uncovered in an archive you're unlikely to run into anyone that knows much beyond general "minimum" standards, which likely evolved and changed over time throughout production. Even if such documentation exists, it is only a glimpse into an individual test that was documented, and may or may not represent the total population of rifles/ammunition produced.
The problem with this question is that there are so many variables at play that it is almost impossible to isolate and quantify the variables. On the surface level you see "K98k" with "Bullet A" in "Ammunition A" (with associated powder, case, primer). The problem is that every one of those things is made up of sub-components, and every one of those sub-components is made up of a raw material or several raw materials and is run through several processes. Each one of the raw materials comes from one or many suppliers/sources and is of variable quality/performance. Each process produces a distribution of end result (Dimensions, tolerances, heat treat, cold work, grain size, forming, hardness, concentricity) etc. The specifications for all of these things change over time (especially in a war). This goes all the way down to the soil content of the trees that were turned into wood pulp to be nitrated to form the nitrocellulose to make the gunpowder, or the specific ore from a specific mine that was used to make the copper or steel strip that was turned into a bullet jacket or cartridge case. This may seem like a deep rabbit hole, because it is. The point is that there is a variation present in every step along the way in the production of the ammunition, as well as the rifle and optic components, and the seemingly simple question of "What is the accuracy produced by XYZ?" is not simple at all. It is the combined dynamic interaction of every piece involved, which is a variable event in itself, and which is subject to all of the variation that occurs in the entire production process leading up to the firing event.
I am an engineer at an ammunition company (Have you noticed?), and we deal with this all the time for government contracts where accuracy requirements are posted. The requirement is Rifle B with Ammunition Y must not produce groups larger than 2 MOA, as an example. The problem is that you can take 50 rifles sequentially made on the same setups with the best components and base materials available, and shoot premium match-grade ammunition that is thoroughly tested for concentricity, weight variation, accuracy potential of the bullet, highest quality powder and primers, and good cases, and you can shoot test groups from those 50 rifles. 30-40 of them will pass the specification easily, 5 of them will flirt with the edge, and the rest will fail. So you have top tier rifles shooting top tier ammunition. Why are there variable results? See the above paragraph. It's the specific interaction between THAT rifle and THAT ammunition and the end result is a variable distribution from rifle to rifle and ammo to ammo. Also, as tooling wears or bullet/powder/primer/case lots change, performance will change. Different lots of "the same" ammo will produce different results. Rifles that initially passed the specification will fail, and rifles that initially failed will pass. Infuriating.
Now add 80 years of variation in maintenance
