I guess i would not say the Lehigh Plug is "opened" up greatly. The biggest factor is that it 'breaths' better as the flash channel has been opened to 5/32". The 'flash hole' has been drilled to .031 from .029. Western Powders recommends a flash hole of .035 tryingto get enought heat to the powder. But that also increases the blow back into the breech plug and the breech that much more.

I am sure that a part Knights original thought process was to open the flash hole to the size that would reliably ignite, at that time T7, yet control the amount of blow back that comes back through the breech plug. Remember in those days T7 was the most difficult powder to ignite and BH was not even heard of...

One of the reasons for using the 25acp, was it was a cooler primer, and that was thought to help reduce the size/amount of the 'crud ring' that T7 placed in the barrel in some rifles. The opposite problem occurs with BH the 'new crud ring' moves from the barrel into the breech plug. I am one that believes the extent of the 'new crud ring' may be related to the temp of the primer. BH is a smokless/progressive burning powder and requires a hight temp ignition source, hence the need to use regular shot gun primers. Western Powders recommends magum primers as the ignition source because of the different breech plugs made. You will find with the Lehigh conversion you will be able to use the coolest 209 primers and achieve ignition. It works that much better.

Even with the Lehigh if you shoot BH you will need to some breech plug maintenance after a shooting a large number shots.

Hope this helps a bit....