.257 Roberts / IMP, case length 2.223" (56.72mm). Seems a long case with Sako mags taking 75mm COL. I wanted to build an RPA in a AICS in 6.5 x 55 using Lapua 139 grain Sc`s and Berger 140 grain VLD`s but the throat would need very little lead and once you start seating the bullet deep you end up better off with .260 REM and long seating the bullets.
I also had the bright idea of 7mm WSM
but the bolt head on the Quadlite in the AICS don`t go.
If .260 REM or .260 REM AI is not exciting a SARUM case in 6.5 may work but the bolt head would need to be opened up and also no Lapua or Norma brass.
In the end I compromised with .260 REM but no compromise with the barrel - cut 5R, your doing it. I get the feeling RPA can and do build actions to suit more or less any case but the limiting factor is the magazine. Hope they have a long action and mag out soon.