Go with the K.
Much more functional and especially with 5.56 the longer barrel allows the bullet to frag as it should within 120-150M (IMI XM855/193) Be that as it may, I also use some steel case lacquered mil-spec 62 gr. SP’s that cycles in the gun well, locks the bolt back and the bullet performs much, much better than FMJ as far as terminal damage. It’s still quite compact and accuracy is as an high grade AR15. Was going to do the C but went K instead but used a C stock.