I might know what the plastic part is.

I once had a Rem 700 .243 with a typical cheap synthetic factory stock, and to test whether bedding would help, I dropped a plastic bread wrapper tag into the recoil lug recess (thanks, MD!) before reinstalling the stock.

It tightened up my groups by half!

Last edited by czech1022; 07/27/16.
All that is necessary for the triumph of evil is that good men do nothing -- Edmund Burke