Quickload and very long freebore

Curious about tricking out quickload for an extremely long freebore.

I have a 458 WM and I loaded up some moderate 300 gr loads with RL7 but the chronoed FPS was down about 300 fps.

It has an extremely long freebore and I would expect that to drop the pressure a lot.

If I add 25% or so to the case capacity it comes close to the chronographed fps.

If I run QL as-is with the correct case capacity to match the FPS with my load and a 21" barrel I have to reduce the barrel length in QL to
about 12" to get the correct FPS