The 55 grain I use is a very frangible bullet, it will come apart on impact. The 65 grain Sierra SBT is a .22 cal bullet developed for hunting larger game like whitetail deer. The jacket on the 65 gr. is also thicker so the bullet will expand but still stay together and achieve better penetration.
The 55 grain will explode on impact even when it hits something small like a gopher, if you hit the shoulder blade on a coyote, the entry hole will be the size of a grapefruit or bigger.