I have a 183-series Mini-14 and if your 181-series wasn't slinging the brass into the next county (province), I would guess that MasterPsmith has nailed it and someone has messed with the gas port with perhaps some buffers thrown in. That can all be fixed.
By the way, recently I took my factory 5-round Ruger magazine that came with the gun to the range just for yucks & giggles, but I couldn't get it to work at all. It's never been used to fire a single round as far as I know. If they will let you have the Ruger 10, 20 or 30 round Ruger Magazines in Canada, I suggest you try some of those and see if that helps your issues.
Good luck with your Mini, believe me they are reliable if you can get the bugs out that may have been installed by "Bubba". Mine has never missed a beat using Ruger 20's and 55 grain bullets.