Not sure how you are measuring , but as said I have two 308AR DPMS Carbines & an extra spring & Buffer , measurements are ;
Spring # 1- 11 1/4 " long
Spring # 2- 11 1/8 " long
Buffer # 1- 2.497 " long ( Measured with Digital Caliper )
Buffer # 2- 2.495 " long ( Measured with Digital Caliper )
Buffer # 1, WT. - 3.325 oz.( weighed with Pact digital Scale )
Buffer # 2, WT. - 3.312 oz.( weighed with Pact digital scale )
I didn't measure the third Spring/Buffer because I didn't think there would be significant differences with it & the others , but will if you want .All are DPMS components .
Small differences can add up when Tolerances are close , as you probably know .
I make springs for Firearms from spring stock , when the springs are no longer available ,( happens a lot ) I cut them & fit & function are trial & error with nothing to go by except a out of spec spring that you know is not working , so cutting a spring is not a big deal .
I would make sure that the Action components are not binding with in the Upper Receiver & its Extension . I would also check for Hammer to Disconnector interference , you will see a mark on the Disconnector from the Hammer hitting it .