Yes, I am bored. So I made an uninteresting and generic "idea" thread.

Assault Carbine - Telescopic Sight (replaces iron sights with ACOG, reduces spread to 0.9)
Assault Carbine - Suppressor (silences gun)
Assault Carbine - Grenade Launcher (makes 40mm usable, separate reload animation)

12.7mm SMG - Stock (reduces spread to 2.75)
12.7mm Pistol - E. Mags (increases capacity to 10)

Assault Carbine - Drum Mag (increase capacity)
Assault Carbine - Reflex Sight

Marksman Carbine - Front Grip (reduces spread)
Marksman Carbine - Silencer

Riot Shotgun - Flashlight

10mm Pistol - Fun Switch (full auto)

Ripper - Tungsten Teeth (increase damage)

Death Claw Mind Controller - have your own

Been playin Black Ops ey?

9mm pistol silencer
marksman carbine night scope
service rifle bayonet

This Machine (yes I know it's unique weapon and can't be modded): Match grade sights, barrel, and trigger group; bayonet.
Sniper Rifle: 10-round mags.
12.7mm SMG: Folding stock and reflex sight.
Riot Shotgun: Folding stock.
Lever-action Shotgun: 12 gauge.

No, Black Ops is floating down the river somewhere. I just want more Assault Carbine attachments to give it a military feel (and improve its terribleness).

I like the Riot Shotgun flashlight idea, although you already have your Pipboy Light. Plus, it hurts your stealth.

AMR Speed Bolt, increases the cycling speed.
Riot Shotgun Choke, decreases spread.
Assault Carbine Masterkey, attaches a 20 Gauge pump-action shotgun to the Assault Carbine, use the ammo-select hotkey to switch to the Masterkey, uses 20 Gauge shotshells.

If you look at the Fallout Wiki it shows a Reflex Sight and Bayonet were cut content for the Service Rifle.

http://images2.wikia.nocookie.net/__cb20110207074929/fallout/images/thumb/7/7d/FNV_Service_Rifle_All.png/830px-FNV_Service_Rifle_All.png

tesla cannon - grounder (chain electricity does not effect allies.)

guass rifle - ion bolts (you recycle one microfusion cell every shot)

gatling laser - coolant (increases capacity)

Light machine gun: hardened barrel +0.4 accuracy and Increase weapon condition by 50%.
Riot shotgun: front grip, decreases spread (this would look beter than a choke IMO).
Combat knife: trench knife mod (like the trench knife from OA).
Assault carbine: silencer, (have been saying this for a long time) if Lilly can have one why can't i :cryvaultboy:.
hunting rifle: repair kit, makes it look more new (without the ductape loose nails and cracks in the wood).