Another idea would be to take a .22 pellet that measures .217 and put into the sizing die and compress it to fill out the space in the die.
That could result in expanding the pellet diameter.
Some of these cylindrical HP pellets have large cavities inside of them that could allow for easy expansion when compressed inside of the die.
It could be worth a try.
If that doesn't work then the pellets can be patched by wrapping them in paper or loose cotton, or by compressing a ball of paper wadding on top of them in the chamber to hold them in place.
The bases may expand when fired like .22LR heeled bullets do.
They should work okay at close range, even if undersized.
But I think that they would work better if they were compressed using the die to try to expand them.
You could also try to wrap the bullets with Teflon tape to get them to hold in the chambers if they were a little loose.
Below are all different brands of .22 slugs but note the large hollow cavity that would help them to fill out the sizing die if compressed in a reverse swagging operation.
The .22 JSB Knockout pellets weigh 25 grains compared to 30-31 grains for the .25 H&N version, and about 27 - 29 grains for a .22 short rimfire bullet.
.22 pellet page at Pyramidair: --->>>
https://www.pyramydair.com/a/Ammo/Airgun_pellets/384/calibers_0_22
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