The sleeve goes all the way back to the breech face inside the bored out portions of your original breeches.

IF the mini-shells do not develop higher pressures than 2.5” cartridges you should not have a problem. Do you know what pressures they produce?

Historically using Charles Lancaster’s Pygmy Cartridges or 2” cartridges in a standard chamber was sometimes found to produce shot balling so pattern testing would be wise.