ronlaughlin
Ron, Builder might be OK, i shoot an even shorter Chedditie primer in my Triumph and really do not experiance much blow back at all in fact very litte. There is a dusting but not much.
This is the breech are in my Triumph after shooting 9 shots of 110 grains of BH and a very short Cheddite primer, it's length averages 0.287"...
This is accomplished by the length of the 'flash channel' in the breech plug. With the 'flash hole' on the end of the Triumph plug the volume capacity of the 'flash channel' is increased to the point where there is very little blow pressure felt on the nose of the primer - there-by reducing blow back