I always wondered why someone didn't make a hybrid round...
Given advances in powder chemistry, the entire volume of a .45
cartridge isn't needed for powder.
Load a reduced volume of more energetic propellant into a case.
Make a gyrojet round such that the back half fits in a .45 ACP case
and has an overall size that matches the normal round.
Design the gyrojet so that the spin matches the normal spin for a .45
round, and that it'll ignite from the powder flash.

The gyrojet is bigger than the normal bullet, but less dense... so
muzzle energy should come out near the same.
However, the jet should be burning as it goes downrange, so you just
keep getting more energy until burnout.