I brought this up about 5 years ago and got hammered then.
I'm still here. It is internal ballistics and not that big of deal.
As long as they are held tight on the charge projectile weight
dictates powder charge.
As long as your not running at the redline let your rifle tell
you if it works.
After all they are nothing more than fancy cannons.
As long as there is no air space psi will be linear with no incursion.
There was also some studies that thought that the powder charge actually acted as part of the projectile weight diminishing as it burned off.
I also remember an article where they filled a section of bbl
with powder and put 2 breech plugs in and lit it with cannon fuse. All the powder burnt and gasses made it out the touch hole.
:surrender: