I am using Blackhorn 209 powder and in most muzzleloaders, it requires a full strength 209 primer like the ones used to reload shotgun shells. The .45 cal Knight DISC Elite that I am using the bare 209 primer conversion breech plug with, cuts down the primer blow back (no scope burn) if I use the Winchester 209 primer because they fit better. I have tried Rem STS, Federal 209A and CCI but they each have just a little difference in depth or width to them.
FWIW, the two inlines that I used T7 in, did just fine with a #11 cap. T7 doesn't require all that a 209 primer has to offer to ignite. My experience has been that the .25 ACP conversion breech plugs work well with T7 using small rifle primers.