Watch the series combat school.
Those are actual milsims.
Once you have, adapt the airsoft tech to that environement and add an element of playability for the players to have a certain degree of fun.
That for me is milsim, having a blast with friends but not compromising proper training and combat techniques and efficiency.
Realistic gear comes second but should always reflect what you would be using in an actual combat situation since proper muscle memory is needed.
Milsim can also be considered closer to sparring in martial arts than gaming shooter style.
There is more to a soldier than just being a good fireing platform for an assault rifle.
Certified Level 2 BA Sniper
Si ton épée est trop courte, allonge la d'un pas. ( Proverbe Hongrois )