I found that the REAR of the compression tube (not the bit that is actually involved in the chamber) is often rough as a badger's arse that has been roughened with badger-paper of the coarsest grade. You can smooth this down with a thick dowel and some fine emery paper, or with a cylinder hone if you are brave and don't get the hones involved with the cocking slot.

Then follow Mr Bum's advice.