I bought a set of 12/10 gauge mates to use in my 10b sb by T Barclay & Son (no relation!). They worked really well with anything with a plastic wad but were hopeless with fibre as I assume too much gas escaped round the wad.
In the UK, most game shoots and many clay grounds are now insisting on fibre wads because of risk to stock/equine so I had to give up their use.
Really should have started loading my own 10b cartridges but too many projects in hand!