Conclusion to the inquiry for whatever it's worth:
Thanks to Darnation who mentioned the idea of a spacer behind the discs!
After measuring the depth of the disc faces comparative to the breech face I found out that they were 5-7 thou proud. Moreso, the ring on the R disc was of a slightly smaller diameter than the L disc ring so that it made pushing a shell base more difficult on the R than on the L. Adding a tighter chamber, everything contributed to a tight closing action.
What I did was to slightly polish the inside ring on the R disc to accept the base of the shell easier. Most of the relief came from filing about 4-5 thou off the back of the discs, at the bottom of the R disc where thickness was 7 thou more comparative to the thinnest part of the discs. It was mainly the R barrel that made the action so tight.
Now, the action is still tight, but much easier to close and open with new rounds...
