28mm bore, a whopping 100mm of stroke, UK spec guns can often have a crazy 2mm port reducer inside. My early .177 52 (1986) has a 3.6mm brass port in the 4mm hole I can't remember TP length, but it's quite short (5 or 6mm ish?) plus a couple of mm through the breech seal...
Yes, cheers Tone I've been researching all the popular faults! Might put up a thread of it's own