I am re-installing a used 308 barrel onto a used action. I never fired this rifle before. I first used a new piece of Lapua brass to headspace it, but when I tried loading a fired case from another 308, it would not chamber. I notice the once fired case (from the other barrel) does not chamber as deep into the barrel as the virgin brass does.

Question: Which piece of brass should I use?

Thanks in advance for any help.