About the only bit of your post I can talk about is the gas pressure. If memory serves, the pressure at the port of the standard M1 is 2,000 psi, from the GI cartridge pressure of 47,000 psi. (If anybody knows differently , please correct me.)
Anyhow, if you load a hotter cartridge, the hole must be smaller. For your 6.5, you could probably safely assume that the ratio of the pressures should be as the ratio of the cross-sectional areas of the gas ports. More pressure = smaller hole.
For a tanker length, the pressure at the gas port in the shorter barrel will be higher than the gas port out at the end of the longer barrel. Same deal as above.
Hope this helps.
, Art