SpudFiles

I know it is not that exiting or inovative but I did a quick search and could not find a bull pup combustion cannon, What I was thinking was that the part of the barrel acutaly Inside the chamber (while still in ratio accounting for the space the barrel takes up) that way you can reload faster(?) without a breach loading system.
Mods feel free to delete if there is a topic already on this

kind of like a coaxial combustion?

i was under the impression that it greatly reduced power... but i'm no combustion expert... *waits for Starman*

Coaxial combustions have less power because the flamefront has to travel around the barrel.
Reloading a coaxial cannon from the back is also considered breach loading btw.
Main advantages:
-Screw open back cap to vent AND load potato in breach. (fast reload)
-Compact
Main disadvantages:
-Longer burn times (less performance)
-Less place for a chamber fan
-Venting is a PITA as there is no free path from back to front as normally as the front is also in the back.

my bad I guess it already existed, just not my terminology was up to date ,

Make it anyway. If you have the end of the barrel inside the gun just a little less, it won't really matter.

And if you really want a fan in there you could just always use a t-joint to put it in.