Are you grouping coyotes into the varmint category? When I think varmints, I think prairie dogs, marmots, jack rabbits, the occasional crow etc..
I have several varmint rifles, none of which burns more than 18 grains of powder. 17AH, 17FB, 20VT and the 221FB. Great choices for volume shooting and easy on powder. If your calling coyotes then a person may want to step up to the 223 but several of the fireball offerings will easily kill a coyote too.