Swapped in the 65mm shim, what a difference! Pretty much shoots to the same POI as my other shotguns, target just over the bead (I much prefer to see the entire target when I pull the trigger). It is a joy to carry, and if I do my part, it reliably does it's part!
Many thanks for all the knowledge sharing on the this forum which helped me personalize my M4. Upgrades:
Collapsable stock, Limbsaver pad, FFT bolt handle, Briley CF mag tube with Nordic extended follower, GG&G bolt release, FFT trigger pack with A&S Trigger guard, TTI extended safety, TTI extended carrier, Carlson breach choke, Tango Arms stippled forearm, and Meprolight night sights.
Awesome. Order placed. In theory I can see how it works, in practice will probably have a learning curve.
Sitting the barrel in a vice so that the barrel hanger At the front is resting on top of the jaws preventing the barrel from shifting downward seems to be the best holding position.