.22 CBee, 33 gr. frangible HP bullet
I have to cycle the action manually, but they go into the chamber from the magazine, which CB Longs wouldn’t do.
I love using the CCI CB Longs in my Marlin 39 - all you hear is the hammer drop, followed by the meaty thump of the bullet connecting with the target. I tried the CB Caps first (same ballistics as the CB Longs, just made on a Short case), but the 39 didn't feed them reliably. I guess even the Short-Long-Long Rifle combo designs have limitations.