I reworked two Mk3s and named them Pepsi and Shirley the former's barrel is shortened by an inch which is just enough to leave a bit of the counterbore. This was then properly finished by Dez the metal polishing guru when all the action was polished to a mirror finish. Craig P chequered both both of their pistol grips and added a dark wood end cap to Shirley where a previous owner's dog chewed it.
Pepsi's stock was stained black then finished with walnut oil, however I found out too late that Beech does not take oil perfectly and I still get a little bit of black on my hands. Don't care about that.
Both have BS 210 quad rings in place of the normal O ring and Shirley sports a new 5.5 mm barrel. I have to home blue her cylinder this winter.
Pepsi has a metal spring guide that came with my Merc S and needed wet & drying to get a good fit, Shirley has a radically cut down Titan spring (240mm) with Titan guide. They both have good firing cycles
The more I think I know, the more I realise the less I know.