Well, as an example, JSB pellets - by and large - don't have as big heads as say H&N FTT and Premiers. You can tell this when you load them into the breech. I've no idea how this equates to nominal head sizes, and as Jon has attested, stated head sizes don't always indicate a particular dimension.
If you can get hold of some samples, try it for yourself. See what pressure you need to get a Barracuda Match into the breech.