Mario,
Where are you getting the breaks cos if they are at the end that you said was damaged even tho youve filed it down really smooth the balls could still break over this piece cos if you think about it whilst youve filed it down you will have removed the layer of coating in the barrel that gives it the final finish and smoothness required to allow the balls to pass through the barrel now seeing as this has been removed in the filled place the balls are now travelling over a piece of rough surface and will break cos of the friction i would try and borrow someone elses barrel and see if it still does it.
Failing that i could be that the paint is maybe abit to brittle and would try a harder ball or the paint hasnt been kept properly and has been effected by the temperatures and would make it nigh on impossible to shoot!!!
Hope this helps mate