Greetings and thank you for reading this!

I inherited (literally) a box of 8mm mauser parts which I was able to reassemble about 95 percent of a rifle from. (Barrel and receiver were not disassembled.)
The previous owner said the rifle shot "good" but I have found the opposite.
With modern ammo (PP 8mm 196gr SPBT), the rifle shoots several inch (like 6 to 8) wide groups at 25 yards.

I have cleaned the barrel literally at least 20 times with all methods I can find.
The bore looks dark (black) and not smooth but the rifling is still visible.
I have cleaned the barrel literally at least 20 times with all methods I can find.
I slugged the barrel and it is in the .323 t0 .324 range and the rifling shows up on the slug.
I did a cast of the chamber and throat an the measurements aren't out of spec.
I have been playing with reloads and lead bullets in it to see if it worked better but it did not.
The crown also appears to be undamaged.

I am wondering if the barrel is shot out or was eroded with corrosive ammo?
The gun is in okay shape but not worth much.
Should I keep working with it or am I wasting my time?
If the barrel is the problem, is it worth rebarreling or should I just hang it on the wall to look at?

Anyone have any input, advice, feedback, comments of a constructive nature?

Thanks in advance for your time and attention!
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