If the bbl. was intended for .22 LR it’s most likely .223” bore dia.
5.5 mm= .216”. Might take a bit to find the best pellet.
@Ronno6 - I just received the .22 barrel liner I ordered from the link you posted and dropped a Daisy HP (Spain version) from each end. The pellet skirt stopped it from going in (approx. last 1/16" sticking out - see pic), so that could be an option for future builds with this specific liner. Unfortunately the barrel came to me with a very slight bend (pic below). Not sure if this is something I could ignore or should I contact the vendor and see about a replacement.