I've got a FV12 in 223 and 204 but just plan on using them from the bench and for PD trips out west. I intend to put boyd's on 'em sometime next year. I don't have any real trouble using the blind mag on these, but usually just do the single shot way as it's just easier IMO. I think trying to use an AR mag would cause more issues than it would be worth, possible feeding issues, the mag will/could limit your options on load development & bullet seating depths, etc.

The 9 twist should shoot the heavy bullets pretty good, mine shoots the 68/69's (my definition of heavy) pretty good for factory ammo, but it seems to prefer the 40-50's in factory ammo better. It will shoot the 75's ok, but I've just started load development for my FV12/223. I suggest you determine what your gun likes the best before switching the barrel, it may like the heavies now, but you don't say.

You don't state what you plan on using this 223 for. If you plan on carrying it around out in the field, its a barrel heavy gun, and would get pretty old in hurry for me anyways. I think the overall design of this model is for shooting from a rest or bipod or bags, whether out in the PD fields, or at the range. That's how I plan on using it anyways.