Re: What weight bullet
300 yd coyote isn't terribly demanding. In that light, white box Winchester 45gr JHP loaded ammo is hard to beat for the money. You can't reload for what those sell for.
Optimum for me is what shoots good in my rifle. And, those get the job done in my 1:12 Steyr and 1:9 dpms which I don't bother to load for.
However, with a 1:9 twist, you should be able to move up to 65-70 gr bullets and develop some pretty decent handloads.
If by "optimum" you mean distance/accuracy, then target bullets will kill coyotes...
Berger 70 gr Match VLD Target
Lapua 69 gr Scenar
69 gr SMK
Hornady 68 gr BTHP