Twilight 2013 (I haven't had any experience with previous twilight versions) is pretty realistic concerning the weapons and how they are handled by the system mechanics, including pretty functional rules for suppressive fire, explosives, morale effects of being in combat using modern weapons, and decent rules for modern armored vehicles in combat. It also has pretty good rules for handling damage (it uses locational damage), the mechanical penalties and effects when injured, and first aid/medicine mechanics.
It has some problems though.
1.) Very crunchy. Not the crunchiest system out there and you can streamline it a lot by making some charts for reference, but the DM and players will need to have a good grasp on how the mechanics work. The book is also not laid out well and as a result the different tables and rules you will need for combat are spread out throughout like half the book. That is why it is very useful for the DM to prepare their own charts and players to make the charts for how their weapon handling at different ranges.
2.) Initiative, or OODA as it is called, is broken. It is far too powerful. It is the most important combat stat by a wide margin. Initiative determines not just who goes first, but how much you can do in your turn. So if you roll really high for initiative, not only are you going first, but you are taking your additional actions before the other guys even get a chance to react. So not only might you be getting 3 attacks compared to their 1 attack, but you are getting at least 2 attacks (and maybe all 3) before they even get a chance to make their 1 attack. Add in the pretty high lethality (or at least high rate of getting incapacitated quickly) you can see how big an advantage that is. It can get pretty extreme with a character built to take advantage of high initiatve. Like a character being able to regularly get off two full 12 rounds bursts with a medium/heavy machine gun in a round before any of the enemies can react (unless you put them up against enemies who are also initiative monsters).
You are gonna want to modify the rules some to reduce how overpowered initiative is. This is what I would recommend, and what I would do if I was gonna run it again. For one you will probably want to not allow the players to get the perks/traits (I forget what they are called in twilight 2013) that makes a character better at using a base stat (initiative is a base stat) with the initiative stat. Second you will probably want to modify it so when a player has a high enough initiative to take multiple actions/attacks, they can't take all of them before the enemy even has a chance to react. They can still go first and they still get their increased number of actions/attacks, but they only get the additional attacks/actions after the enemies with fewer actions get a chance to do something.
3.) The rules don't really handle sneaking/concealment in combat. They have mechanical rules for handling sneaking and spotting people trying to sneak, including the rules for noticing an ambush, but the combat mechanics mostly assume that once they are in combat everyone involved in the fighting is aware of where all the enemies involved in the fighting are. This is a lot easier to modify the rules for though as you mostly just need to apply the rules for sneaking/being spotted during combat. The system even already gives you the tables for how the noise of gunfire affects the attempt to remain concealed (although I don't think it has any specific rules for how the visual signature of firing a weapon affects concealment so you may have to make up the numbers for that).
