As mentioned,try cleaning it completely.If disassembly might be an issue, take it to a good gunsmith and have it done.I had one myself years ago and took it apart out of curiousity but couldnt get it to go back together.A smith charged me 10 bucks to reassemble it and showed me how that bolt thing went back in there and how everything worked together in the trigger and the tube magazine,well worth it to understand it..
It shouldnt have a gritty trigger.Mine broke pretty clean even when it was dirty,never cleaned it in 5 years other than a patch down the bore.Something doesnt sound right.