I bet you are all thinking “Oh noes, its gonna become cod and gimmicky”. But let me explain. This would explain how you can get the pistol projectile and scope.
So lets get things started.
When single wielding a dual wield-able weapon, you get less bloom+spread (if not no bloom+spread) and add use to attachments. When dual wielding, Attachments are disabled and bloom+spread is increased. This makes small weapons like these useful in many different ways. Dual wielding is best for close range and high damage and single wielding is good for range and customisable tactics.
Now that I got that out of the way. THIS is what the attachment system is. Its the ability to modify your weaponry from buying “attachments” with the credits earned in-game. You have a limit of two attachments at a time. There would be an expensive upgrade where you can have up to three.
THERE WILL AN ABILITY TO DISABLE ATTACHMENTS THROUGH THE GAME OPTIONS IF THIS IS EVER DONE.
Right. Here are the weapon attachments you an make to upgrade your weaponry. This is just the list for the human weaponry:
Assault Rifle:
*Holographic sight - adds a scope to your AR for long ranged shooting.
*Re-enforced grip - Decreases recoil and spread when firing.
*Longer clip - Takes the 32 round grip back to the classic 60 bullet magazine.
*Extra Calibre - Does more damage and makes the classic halo 1 AR sound.
*Rapid fire - Fires twice the speed but in expense of wasting your ammo quickly.
Pistol:
*Scope - Adds a scope. durr
*Silencer - Makes your shots quiet for stealthy tactics. Much like the ODSTs!
*Anti-recoil - Dramatically reduces recoil and bloom if not gets rid of it.
*Extra Calibre - Beefs up the power and deals almost twice the damage.
*Rapid-fire - Turns it into a machine pistol. This however adds more bloom, wastes bullets and takes down the damage of each shot.
*Secondary projectile - This is what you have seen in the Halo 4 trailer! Fires a single explosive from the gun temporally replacing the grenade button. Can be picked up from fallen player who have it. Can be upgraded to carry two at once.
SMG:
*Silencer - Makes your shots quiet for stealthy tactics. Much like the ODSTs!
*Holographic sight - adds a scope to your SMG for long ranged shooting.
*Shoulder back grip - Decreases recoil and bloom.
*More rounds - Adds more bullets to the magazine.
*Grenade launcher - Behaves the same way the projectile from the pistol does. But is instead a cut down and uncontrollable version of the grenade launcher. Does just a single shot for those last minute actions.
BR (if its back):
*Anti-recoil - Adds less bloom.
*Faster bursts - Fires each burst dramatically faster, causing more damage.
*Larger magazine - Fires more bullets before reloading.
*Heat-sensitive scope - Clearly see active-camouflaged or hidden enemies through walls. They show up reddish-yellow on the sight.
DMR (if its back):
*Faster shots - Shortens intervals between shots.
*Anti-Recoil - Adds less bloom.
*Larger magazine - Fires more bullets before reloading.
*Extra Caliber - Deals more damage per shot.
*Heat-sensitive scope - Clearly see active-camouflaged or hidden enemies through walls. They show up reddish-yellow on the sight.
Sniper Rifle:
*Heat-sensitive scope/Night Vision - Clearly see in the dark. Also see active-camouflaged or hidden enemies through walls. They show up reddish-yellow on the sight.
*Longer Scope - As well as 5x and 10x scope. There is an extra 20x scope that can take out people from across the map. Bullet drag may be added to prevent camping.
*Extra Caliber - Deals more damage per shot.
*Larger magazine - Fires 2 more bullets before reloading.
Rocket Launcher:
*Faster Reload - Every get frustrated when trying to reload your rocket launcher? This would solve all your problems!
*Blast Radius - Adds more area of effect and make anything near by more likely to fly off!
*Focused Damage - Anything in the centre of the explosion would take more damage.
*Back-up ammo - Adds and extra 2 rockets than what it is spawned with.
Spartan laser:
*Faster Charge - Takes less time to charge up a shot
*Focused Damage - Anything in the centre of the beam would take more damage.
*Larger radius - Creates a wider beam that effects more things around it. Much like the halo 3 one.
*Coolant - Speeds up the after cooling process before firing again.
*Efficient power - Reducing the power used by each shot. Intentionally adding 2 extra shots.
Shotgun:
*Faster reload - Speeds up both the shot interval and reload speed.
*Ranger - Takes the range up to the halo ce standard. Carries the shot twice the distance! Adds the decorative from the halo reach shotgun.
*Extra Caliber - Deals more damage per shot.
*Larger magazine - Fires more shots before reloading.
*Larger spread - For people who don’t like aiming!
Grenade Launcher (if its back):
*Faster reload - Pops in the next explosive a lot quicker than before!
*Explode on impact - No bouncing.
*Sticker - Sticks to the wall (or to someone else).
*Blast Radius - Adds more area of effect and make anything near by more likely to fly off!
*More Emp - Takes down shields and vehicles on impact as well as explosive.
*Multi-shot - Shoot as many as you like. The grenade button is temporally replaced with a detonation button when at least one of the explosives are fired.
Target Locater (if its back):
Quick response - Your fire team with respond almost immediately to your command.
Quick change - Your fire team will reload their weapons faster.
More explosions - The bombing run will last longer on your target.
Extra Commands - You can fire this an extra two times!
These are just for the human weapons. I may release the ones for the covenant/forerunners weapons but I’m guessing you have the right idea about it already! 
Please tell me what you think!