First post for me in this forum, and I would like to start by thanking everyone for writing such entertaining and informative threads, especially in my case regarding the Mannlicher Schönauer 1910, which I recently came to own. Without this forum I would be completely lost.
Please bear with me for a quick introduction before my questions;

I have recently been given an Mannlicher Schönauer 1910 in the 9,5x57MS caliber. It was given to me by my father, who long ago inherited it from his grandfather.
It a full stock in great condition, and the only modification is a small silver plaque embedded in the stock with my great grandfathers name engraved on it. However, it has once been "disabled" due to Swedish gun laws (when my great grandfather died no one in my family had permits to own firearms) and this was done by welding a bolt into the chamber. When my father got his permits in order he left the rifle to a gunsmith to remove the bolt. According to my father, the gunsmith then used something similar to a dentist drill to remove the residue from the welding. My father only used it a couple of times after that in the 80's, and since then it has been retired to his gun closet.
It was recently given to me because my father had not fired it for 20 years or so, and has always had a hard time finding ammunition for it.
I recently started handloading. I'm still a novice, but starting to learn a trick or two. Therefore the rifle was passed on to me, to see if I could start making ammunition and maybe even use it for hunting in the future.

This is how far I've come at this point. I have, after many more or less failed attempts, started forming cases from S&B 30-06 brass, which luckily is not hard to find here, people throw it away like trash. These cases works like a charm, and after some experimenting with loads I recently got to try it out for real on the range. It was actually the first time I used iron sights, and I was pleasantly surprised when I got around 2" groups at 100 yards with an occasional flyer or two.
I am using the .375 Hornady Interlock 270gn RN, however, I am only getting velocities up towards 2000 fps before cases start to grip the chamber really hard, the bolt then needs a serious beating to open, and you can clearly see some markings in the brass from what I can only assume to be leftovers from the welding I mentioned before.
I'm not completely sure that the welding damage is the only reason that the case gets stuck when only achieving such low speed, since I discovered another odd thing while developing a load.
I first tried, as recommended by another forum, to fireform 9,3x57 cases by fitting a bullet to have contact with the rifling. This is not possible, the chamber seems to be too long to do this.
I fitted a bullet loosely and chambered it, and it did not even touch the rifling. I did a (highly unscientific) measurement with a cleaning rod, and concluded that the bullet have about 1-2mm "free flight" before hitting the rifling.

Does my rifle have an abnormally large chamber? Could a large chamber cause a loss in performance that can explain the low speed even though the cases indicate high pressure?

My other question is; could a competent gunsmith even out the chamber? My guess from looking at the cases is that the aforementioned gunsmith took to much material out at some places in the chamber, and too little at other places. I imagine that fixing the spots where there is too much material won't be a problem, but how about where material needs to be "added", is this even possible?
Will it be worth trying, or is it ruined?

I apologize in advance if I use some weird/incorrect technical terms, this is my first time writing about these sort of things in English.