A 14mm sleeve would be a good choice.... Note on that Index you can also get a 10mm ID x 15mm OD.... You can also order TJ's liners with a larger OD, I have a .172 cal with a 14mm OD, so I know they can do that.... In fact, they have a short sample of 14mm ID CF tubing I sent them to check the OD while they are hammer forging it.... The right hand barrel in the photo above is the .172 cal TJ's with a 14mm OD.... Before you order anything, talk to TJ's about barrel OD's available.... Sleeving to 15.5 mm so that a stock barrel will fit is a good idea, but of course if you had a 16mm OD sleeve, you could turn that down to fit the breech.... Bob