Ever hear of the 80 - 20 rule?
80 percent of the results come at 20 % of the effort,
and that last 20 % costs you 80 %.
IMHO,
shims are a complicated solution to a non-existent problem. Shimming the gas assembly is about 1 % of what makes an M-14 accurate. And you can achieve the same results with a machinest spring loaded center punch, and a few minutes of judicious staking in the correct locations.
A good USGI fiberglass stock, a GI bolt properly fitted, and a decent trigger job will improve your M-14 rifle's practical accuracy about as far as most people ever need to go. After that, the point of diminishing returns has been reached ...
BTDT myself more than once
[;{)
LAZ 1