then make it! but really, on a realistic standpoint, take an existing system and modify it to suit your needs.
that's what I normally do.
That's what I'm doing! I've been talking with friends and we're pretty sure we've got the ranged combat system done.
Each weapon fits into one of three types: Assault, Sidearm or Heavy
Each weapon fits into one of three range-categories: Long-Ranged, Mid-Ranged or Short-Ranged
When firing the weapon, you add your weapon type bonus to your range bonus. A weapon can be fired in shorter ranges than its range category, but not further ranges. Your range bonus is calculated by adding the weapon's range category to the actual range and dividing by two.
Take an example! We'll make up a random Spartan... Let's call him John. John has the following stats:
John wants to fire his Sniper Rifle (Long-range, assault) at an Elite standing 10 feet away, which is short range.
John's short range skill (3) + his Long-range skill (4) = 7
Divide by two: 7 / 2 = 3.5 which we round up to 4
John's total range bonus is +4
He has a +4 to assault weapons
John gets a total of +8 on his roll.
Now this might seem like a pain in the arse to work out all the time, but you can construct a simple table to refer to. In fact, I already have.
It didn't occur to me at the time that the bottom left 3 boxes should be left out as you can't fire a shotgun at long-range (as an example). This means that you just have to refer to the table, which should be pretty easy, and adding two numbers together (three with your roll).
Melee combat hasn't been addressed yet.