Here are some very rough descriptions of the backbores:

 

As you can see the KA2 opens up much more quickly after the throat.

 

The KA1 opens up ok, but stays tight through almost to the end.

 

The KA3 opens up like KA1, but then opens up again like the KA2 at the end.  It has for me,  the most compression of the three backbores making it feel about one throat size smaller overall.

 

 

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