I have yet to see (or hear) of a test where peak pressure wasn't reached within a short distance after the bullet left the case and entered the rifling. Say 2" on average.
Beyond that the pressure starts dropping, due to the greater volume inside the bore behind the bullet. Have yet to see/hear of any test that showed a significantly higher pressure peak due to a faster twist. The biggie is the pressure required for the bullet to enter the rifling.
