I have a remi in 223, its a 1 in 10 twist, I load 69gn mainly for target, the first ones i loaded wouldnt stabilise, so i needed to up the powder to 24-5 gns of N135, this is about (slightly above) max recomendations, however it will shoot rather well at 300meters even with me behind it. You should be able to get up to about 80+ gn bullets with a 1:8, but i guess you will need to make sure they are accurate and fast enough . I find for accuracy at over 100meters the heavier the bullet , the less the wind moves it, the downside is heavier bullets dont fly so flat so you need to be more aware of the rise/fall of flight path.
For vermin you wont go wrong with 55gn Vmax. If you dont load your own , just try differnt weights and see where you are. With modern bullets, personally i would go for accuracy first, and then choose the type of bullet to do the job, a 55gn vmax does more damage to a fox than a 69gn non-expanding, although i wouldnt think a 223 is suitable for anything much bigger than a fox in the UK, to meet policeforce guidlines, good luck !