Here is a simple render of an interesting project. Ultimately, this didn't actually didn't result into a muzzle device, but was machined directly into the barrel. The sleeve took the look of a bell, and so the project was deemed "blunderbuss".
The logic behind this part was to reduce and redirect muzzle pressure from behind the bullet, before the bullet left the muzzle, thereby providing a smoother transition.
It was scrapped and put aside before extensive testing was done. It did not seem to have any negative impact on accuracy though. Perhaps I'll revisit it.