The "K" is usually said to stand for "Kilbourn" as in Lysle Kilbourn the originator of the most popular of the improved Hornet's... it is not a "modern" shape as stated above as it is a very old design (Kilbourn died in '68). Any Hornet chambered can be opened up to a "K" very simply... I do all my rechambers by hand with a live pilot reamer and a "go-gauge"... the whole process takes 30 minutes. Any Hornet ammo can be fired in a "K" chamber... which is how brass is made, by simply fire forming Hornet brass. I use Winchester Hornet ammo and simply shoot it in the K-chamber... then subsequently load the brass with a RCBS K-H FL die and seater... the reamer is from Dave Kiff at Pacific Tool... the beauty of the "K" is the ease of chambering, forming brass and improved brass life... I net 150-200 fps over the standard Hornet depending on load. It is a fun loading the little pills and a joy to shoot.