I have used the 11mm betains and 18mm halibuts from ccmoores and they both sometimes split Richard.
I think sometimes the way that I drill makes a difference, if I rush the procedure I think the hole can be a little wider and they split.
That said, other factors I think also make a difference such as how old the pellets are and if they have been frozen, glugged etc but, I havent quite sussed out what exactly makes a difference, as yet.
I tend to drill a load at once and put these in a separate container so as I always have drilled hookbaits to hand.