I did a little more reading on the Gyrojet. The original pistol ammo was 13mm wide and pretty long
. That's how they got enough rocket fuel in it
. Then they realized that 13mm was bigger than 50cal so they couldn't sell it to civvies, and produced a 12mm version...
Hmmmm. Ya, on top of everything else, we ain't gonna get enough boost out of a .32. Or even a 45ACP shell set up as a projectile - too short.
Oh well.
(scratches head)
But...here's a question: I already suspected that additional pressure kicking in when the round was halfway down a 5" or so barrel wouldn't be a problem.
So...what if we *used* that? Take the whole concept of "duplex load" to the next level?
We know powder can be set up as a hard, dry "block". That's how caseless ammo works, right? Or Pyrodex pellets for the black powder crowd, although that's not quite the powder type we want. Still, the principle applies.
OK, take a...hey, I know what we can use for development anyhow, a 148grain .357cal hollow base wadcutter! It's got a big ol' cavity in the back. Fill that with some some sort of powder paste that is going to be dried prior to loading. Figure out a way to delay the ignition of it until it's partway down the barrel! How? Hmmm...layer of rice paper? Layer of thicker paper with a fuse in the middle? Needs more study. But you can make up "experimental controls" that are filled with a non-flammable dried substance of some sort, and chrono those in comparison to the "second stage active" variety, so you can see if you're getting a real "second stage effect".
If it works, "supersize it" by taking a .452 caliber 500grain cast slug, and boring out the tail until it's down to around 250-300 grains, fill the rear with another "second stage load" and use it in a strong .45LC gun.
Peak pressure should be low for the velocity/ME achieved, and recoil energy would be "spread out" across a longer period?