For the load developement of bowhunthards gun, I'm not sure how much reloading you've done but.....As you go up in the load weight, you will see the group tighten up and as you go higher in load weight the group will widen then tighten back up again. This will happen over and over until you reach a maximum charge. You just have to figure out when you want to stop and accecpt what you have. Also, when working with small calibers it's best to go up 1/10 of a grain at at time. Don't skip and try 1/2 grain increments or something like that. You could pass right over your best group and never even know so always go 1/10 grain at a time. Just my .02.
A 12 twist will never stablilze a 75gn A-Max. I shoot them in my 22/257 with a 1 in 8 twist and it's just enough. I built the gun to shoot the 80gn A-Max but even with the 1 in 8 twist, they just key-hole. Everything on line says the 1 in 8 is all thats needed to shoot 80gn bullets but mine won't. You need no less than a 1-9 twist to shoot the 75grainers.
A 1 in 12 in .224 should handle up to 68-69 gn maybe, but that would be the most it would handle. It probably would love the 50-65gn bullets the best.
Hope this helps.