I am new to coyote hunting & have a factory .243 Win chambered Rem 700 (1 in 9.125" twist) that I am currently using. Considering going to a .223 Rem chambered Howa Mini (1 in 8" twist) shooting 77 gr Sierra MK hollow points. Is that a decent bullet to anchor them @ 300 yards if they hit in the boiler room ?
I wouldn't bet on a 77 grain MK expanding so you're truly asking whether a .22 cal FMJ will "anchor." I'd say not reliably. It will kill .. eventually .. but without breaking major bones, you probably won't find them where you shot them, they'll run a ways before leaking enough to run out of oxygen to the brain. I would say the inexpensive option is the Hornady 55 or 60 grain soft point. Otherwise 60 grain VMAX or Ballistic tip.