Well I've been playing around with one of my BSA Challengers today and thought I'd measure the transfer port whilst the barrel was off, well the length is 13mm and diameter is 3.3mm, I bought some extra long drills the other day ranging from 3mm to 4mm., so it would be interesting to see how efficient those measurements are for that rifle.
Pete
Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in
No mention of calibre...does it not influence the optimum size of TP required?
Get yourself a cheap pin vice and hammer a long steel rod up the back end. Make sure you wrap the widest part in tape so it doesn't scratch the inside of the cylinder.