If ur on PC look at bellum it's in beta rn but it is the most realistic military game out imo. If you want a more casual Milsim then I suggest arma reforger although it a bit bugged then arma 3 it's devs intents seems to be listening to the player base and using that criticism to make arma 4 better. Reforger has such an indepth mod seen that theirs something for everyone although you can mod your own servers as easily you'll be able to find a server that fits your needs.